With Numlock Off:
-------------------------------------------------------------------------------
KeyPress event, serial 36, synthetic NO, window 0x4200001,
    root 0xbd, subw 0x0, time 542002, (434,589), root:(488,641),
    state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 36, synthetic NO, window 0x4200001,
    root 0xbd, subw 0x0, time 542098, (434,589), root:(488,641),
    state 0x8, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False
-------------------------------------------------------------------------------

With Numlock On:
-------------------------------------------------------------------------------
KeyPress event, serial 36, synthetic NO, window 0x4200001,
    root 0xbd, subw 0x0, time 608978, (1048,476), root:(1102,528),
    state 0x10, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 36, synthetic NO, window 0x4200001,
    root 0xbd, subw 0x0, time 609082, (1048,476), root:(1102,528),
    state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False
-------------------------------------------------------------------------------

The only difference I can see is the extra bit set in the value of 'state'. Just to be sure, I reran my steps, and I can still reproduce the problem on a clean reboot. My keyboard is a Dell USB model. I've attached the relevant output of lsusb in case it matters.

-------------------------------------------------------------------------------
Bus 005 Device 002: ID 413c:1004 Dell Computer Corp. 
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            9 Hub
  bDeviceSubClass         0 Unused
  bDeviceProtocol         0 Full speed (or root) hub
  bMaxPacketSize0         8
  idVendor           0x413c Dell Computer Corp.
  idProduct          0x1004 
  bcdDevice           48.01
  iManufacturer           1 Dell
  iProduct                2 Dell USB Keyboard Hub
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           25
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          2 Dell USB Keyboard Hub
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         9 Hub
      bInterfaceSubClass      0 Unused
      bInterfaceProtocol      0 Full speed (or root) hub
      iInterface              2 Dell USB Keyboard Hub
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0001  1x 1 bytes
        bInterval              24
Hub Descriptor:
  bLength               9
  bDescriptorType      41
  nNbrPorts             3
  wHubCharacteristic 0x000d
    Per-port power switching
    Compound device
    Per-port overcurrent protection
  bPwrOn2PwrGood       22 * 2 milli seconds
  bHubContrCurrent    100 milli Ampere
  DeviceRemovable    0x02
  PortPwrCtrlMask    0xff
 Hub Port Status:
   Port 1: 0000.0303 lowspeed power enable connect
   Port 2: 0000.0100 power
   Port 3: 0000.0100 power
Device Status:     0x0000
  (Bus Powered)

Bus 005 Device 003: ID 413c:2006 Dell Computer Corp. 
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0         8
  idVendor           0x413c Dell Computer Corp.
  idProduct          0x2006 
  bcdDevice           48.00
  iManufacturer           1 Dell
  iProduct                2 Dell USB Keyboard Hub
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           59
    bNumInterfaces          2
    bConfigurationValue     1
    iConfiguration          3 83NT6504V110
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower               90mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      1 Boot Interface Subclass
      bInterfaceProtocol      1 Keyboard
      iInterface              2 Dell USB Keyboard Hub
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.10
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      65
         Report Descriptors: 
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval              10
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              2 Dell USB Keyboard Hub
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.10
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength     142
         Report Descriptors: 
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0004  1x 4 bytes
        bInterval             255
Device Status:     0x0000
  (Bus Powered)
-------------------------------------------------------------------------------