Event XML just before it re-fired (after previously dismissed):


        <m:GetItemResponseMessage ResponseClass="Success">
          <m:ResponseCode>NoError</m:ResponseCode>
          <m:Items>
            <t:CalendarItem>
              <t:ItemId Id="AAMkADQ4YzA4OWJkLTc1NGItNGE2OC1hYjMxLWNlNDgyM2NiNjg2MABGAAAAAAA1rOHRhKF2TqddRoRRd4o2BwC8rol0GelCSbBtWoxwGP1qACyQoE8lAACYqrrsDVjYTYWNLzzK3tSyAAO5RJ7lAAA=" ChangeKey="DwAAABYAAACYqrrsDVjYTYWNLzzK3tSyAAO6iHm3"/>
              <t:Subject>test</t:Subject>
              <t:Body BodyType="Text"></t:Body>
              <t:DateTimeReceived>2011-11-23T10:25:21Z</t:DateTimeReceived>
              <t:ReminderDueBy>2011-11-23T10:30:00Z</t:ReminderDueBy>
              <t:ReminderIsSet>true</t:ReminderIsSet>
              <t:ReminderMinutesBeforeStart>0</t:ReminderMinutesBeforeStart>
              <t:ExtendedProperty>
                <t:ExtendedFieldURI DistinguishedPropertySetId="Common" PropertyId="34144" PropertyType="SystemTime"/>
                <t:Value>2011-11-23T10:30:00Z</t:Value>
              </t:ExtendedProperty>
              <t:ExtendedProperty>
                <t:ExtendedFieldURI DistinguishedPropertySetId="Meeting" PropertyId="3" PropertyType="Binary"/>
                <t:Value>BAAAAIIA4AB0xbcQGoLgCAAAAABwS6A0yqnMAQAAAAAAAAAAEAAAAEzHkDDZw1lHg8CPX/N5Rsw=</t:Value>
              </t:ExtendedProperty>
              <t:UID>040000008200E00074C5B7101A82E00800000000704BA034CAA9CC010000000000000000100000004CC79030D9C3594783C08F5FF37946CC</t:UID>
              <t:Start>2011-11-23T10:30:00Z</t:Start>
              <t:End>2011-11-23T11:00:00Z</t:End>
              <t:IsAllDayEvent>false</t:IsAllDayEvent>
              <t:Location/>
              <t:IsCancelled>false</t:IsCancelled>
              <t:CalendarItemType>Single</t:CalendarItemType>
              <t:MyResponseType>Organizer</t:MyResponseType>
              <t:Organizer>
                <t:Mailbox>
                  <t:Name>Marsh, Kris</t:Name>
                  <t:EmailAddress>myemail</t:EmailAddress>
                  <t:RoutingType>SMTP</t:RoutingType>
                </t:Mailbox>
              </t:Organizer>
              <t:RequiredAttendees>
                <t:Attendee>
                  <t:Mailbox>
                    <t:Name>Marsh, Kris</t:Name>
                    <t:EmailAddress>myemail</t:EmailAddress>
                    <t:RoutingType>SMTP</t:RoutingType>
                  </t:Mailbox>
                  <t:ResponseType>Unknown</t:ResponseType>
                </t:Attendee>
              </t:RequiredAttendees>
            </t:CalendarItem>
          </m:Items>
        </m:GetItemResponseMessage>



---
Then after dismissing at Outlook then restarting Thunderbird (event appears at least twice in the debug log,  below are the first and last):


        <m:GetItemResponseMessage ResponseClass="Success">
          <m:ResponseCode>NoError</m:ResponseCode>
          <m:Items>
            <t:CalendarItem>
              <t:ItemId Id="AAMkADQ4YzA4OWJkLTc1NGItNGE2OC1hYjMxLWNlNDgyM2NiNjg2MABGAAAAAAA1rOHRhKF2TqddRoRRd4o2BwC8rol0GelCSbBtWoxwGP1qACyQoE8lAACYqrrsDVjYTYWNLzzK3tSyAAO5RJ7lAAA=" ChangeKey="DwAAABYAAACYqrrsDVjYTYWNLzzK3tSyAAO6iHm3"/>
              <t:Subject>test</t:Subject>
              <t:Body BodyType="Text"></t:Body>
              <t:DateTimeReceived>2011-11-23T10:25:21Z</t:DateTimeReceived>
              <t:ReminderDueBy>2011-11-23T10:30:00Z</t:ReminderDueBy>
              <t:ReminderIsSet>true</t:ReminderIsSet>
              <t:ReminderMinutesBeforeStart>0</t:ReminderMinutesBeforeStart>
              <t:ExtendedProperty>
                <t:ExtendedFieldURI DistinguishedPropertySetId="Common" PropertyId="34144" PropertyType="SystemTime"/>
                <t:Value>2011-11-23T10:30:00Z</t:Value>
              </t:ExtendedProperty>
              <t:ExtendedProperty>
                <t:ExtendedFieldURI DistinguishedPropertySetId="Meeting" PropertyId="3" PropertyType="Binary"/>
                <t:Value>BAAAAIIA4AB0xbcQGoLgCAAAAABwS6A0yqnMAQAAAAAAAAAAEAAAAEzHkDDZw1lHg8CPX/N5Rsw=</t:Value>
              </t:ExtendedProperty>
              <t:UID>040000008200E00074C5B7101A82E00800000000704BA034CAA9CC010000000000000000100000004CC79030D9C3594783C08F5FF37946CC</t:UID>
              <t:Start>2011-11-23T10:30:00Z</t:Start>
              <t:End>2011-11-23T11:00:00Z</t:End>
              <t:IsAllDayEvent>false</t:IsAllDayEvent>
              <t:Location/>
              <t:IsCancelled>false</t:IsCancelled>
              <t:CalendarItemType>Single</t:CalendarItemType>
              <t:MyResponseType>Organizer</t:MyResponseType>
              <t:Organizer>
                <t:Mailbox>
                  <t:Name>Marsh, Kris</t:Name>
                  <t:EmailAddress>myemail</t:EmailAddress>
                  <t:RoutingType>SMTP</t:RoutingType>
                </t:Mailbox>
              </t:Organizer>
              <t:RequiredAttendees>
                <t:Attendee>
                  <t:Mailbox>
                    <t:Name>Marsh, Kris</t:Name>
                    <t:EmailAddress>myemail</t:EmailAddress>
                    <t:RoutingType>SMTP</t:RoutingType>
                  </t:Mailbox>
                  <t:ResponseType>Unknown</t:ResponseType>
                </t:Attendee>
              </t:RequiredAttendees>
            </t:CalendarItem>
          </m:Items>
        </m:GetItemResponseMessage>
      </m:ResponseMessages>
    </m:GetItemResponse>
  </soap:Body>
</soap:Envelope>











        <m:GetItemResponseMessage ResponseClass="Success">
          <m:ResponseCode>NoError</m:ResponseCode>
          <m:Items>
            <t:CalendarItem>
              <t:ItemId Id="AAMkADQ4YzA4OWJkLTc1NGItNGE2OC1hYjMxLWNlNDgyM2NiNjg2MABGAAAAAAA1rOHRhKF2TqddRoRRd4o2BwC8rol0GelCSbBtWoxwGP1qACyQoE8lAACYqrrsDVjYTYWNLzzK3tSyAAO5RJ7lAAA=" ChangeKey="DwAAABYAAACYqrrsDVjYTYWNLzzK3tSyAAO6iVOR"/>
              <t:Subject>test</t:Subject>
              <t:Body BodyType="Text"></t:Body>
              <t:DateTimeReceived>2011-11-23T10:25:21Z</t:DateTimeReceived>
              <t:ReminderDueBy>2011-11-23T10:30:00Z</t:ReminderDueBy>
              <t:ReminderIsSet>false</t:ReminderIsSet>
              <t:ReminderMinutesBeforeStart>0</t:ReminderMinutesBeforeStart>
              <t:ExtendedProperty>
                <t:ExtendedFieldURI DistinguishedPropertySetId="Common" PropertyId="34144" PropertyType="SystemTime"/>
                <t:Value>4501-01-01T00:00:00Z</t:Value>
              </t:ExtendedProperty>
              <t:ExtendedProperty>
                <t:ExtendedFieldURI DistinguishedPropertySetId="Meeting" PropertyId="3" PropertyType="Binary"/>
                <t:Value>BAAAAIIA4AB0xbcQGoLgCAAAAABwS6A0yqnMAQAAAAAAAAAAEAAAAEzHkDDZw1lHg8CPX/N5Rsw=</t:Value>
              </t:ExtendedProperty>
              <t:UID>040000008200E00074C5B7101A82E00800000000704BA034CAA9CC010000000000000000100000004CC79030D9C3594783C08F5FF37946CC</t:UID>
              <t:Start>2011-11-23T10:30:00Z</t:Start>
              <t:End>2011-11-23T11:00:00Z</t:End>
              <t:IsAllDayEvent>false</t:IsAllDayEvent>
              <t:Location/>
              <t:IsCancelled>false</t:IsCancelled>
              <t:CalendarItemType>Single</t:CalendarItemType>
              <t:MyResponseType>Organizer</t:MyResponseType>
              <t:Organizer>
                <t:Mailbox>
                  <t:Name>Marsh, Kris</t:Name>
                  <t:EmailAddress>myemail</t:EmailAddress>
                  <t:RoutingType>SMTP</t:RoutingType>
                </t:Mailbox>
              </t:Organizer>
              <t:RequiredAttendees>
                <t:Attendee>
                  <t:Mailbox>
                    <t:Name>Marsh, Kris</t:Name>
                    <t:EmailAddress>myemail</t:EmailAddress>
                    <t:RoutingType>SMTP</t:RoutingType>
                  </t:Mailbox>
                  <t:ResponseType>Unknown</t:ResponseType>
                </t:Attendee>
              </t:RequiredAttendees>
            </t:CalendarItem>
          </m:Items>
        </m:GetItemResponseMessage>


First occurrence one looks like the previously cached (identical to pre-dismissal) event?

Outlook seems to:
* Update item ID
* Set ReminderIsSet=false
* Set ExtendedProperty->SystemTime = 4501-01-01T00:00:00Z


<t:ItemId Id="AAMkADQ4YzA4OWJkLTc1NGItNGE2OC1hYjMxLWNlNDgyM2NiNjg2MABGAAAAAAA1rOHRhKF2TqddRoRRd4o2BwC8rol0GelCSbBtWoxwGP1qACyQoE8lAACYqrrsDVjYTYWNLzzK3tSyAAO5RJ7lAAA=" ChangeKey="DwAAABYAAACYqrrsDVjYTYWNLzzK3tSyAAO6iVOR"/>

<t:ReminderIsSet>false</t:ReminderIsSet>

              <t:ExtendedProperty>
                <t:ExtendedFieldURI DistinguishedPropertySetId="Common" PropertyId="34144" PropertyType="SystemTime"/>
                <t:Value>4501-01-01T00:00:00Z</t:Value>



Hope that helps!