I'm also seeing this on a Jaunty to Karmic upgrade.
I've ran the icon-cache update tool, that I read from Google searches, no change.
Once I click the 'OK' on the prompt (I get three prompts) then my network connectivity stops.

If I run `nm-applet` from the command line I see this:
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** (nm-applet:2246): WARNING **: Icon nm-signal-00 missing: Error opening file: No such file or directory
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From strace I've learned this:
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lstat64("/usr/share/icons/elementaryXubuntu/apps/48/nm-signal-00.svg", {st_mode=S_IFLNK|0777, st_size=15, ...}) = 0
stat64("/usr/share/icons/elementaryXubuntu/apps/48/nm-signal-00.svg", 0xbfdcd36c) = -1 ENOENT (No such file or directory)
readlink("/usr/share/icons/elementaryXubuntu/apps/48/nm-signal-00.svg", "nm-signal-0.svg", 256) = 15
open("/usr/share/icons/elementaryXubuntu/apps/48/nm-signal-00.svg", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)
** (nm-applet:2632): WARNING **: Icon nm-signal-00 missing: Error opening file: No such file or directory
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And if I look for those files, I can see their links are broken
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root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ls -alh nm*
lrwxrwxrwx 1 root root 15 2009-10-16 22:13 nm-signal-00.svg -> nm-signal-0.svg
lrwxrwxrwx 1 root root 54 2009-10-16 22:13 nm-signal-0.svg -> ../../status/48/notification-network-wireless-none.svg
lrwxrwxrwx 1 root root 54 2009-10-16 22:13 nm-signal-100.svg -> ../../status/48/notification-network-wireless-full.svg
lrwxrwxrwx 1 root root 53 2009-10-16 22:13 nm-signal-25.svg -> ../../status/48/notification-network-wireless-low.svg
lrwxrwxrwx 1 root root 56 2009-10-16 22:13 nm-signal-50.svg -> ../../status/48/notification-network-wireless-medium.svg
lrwxrwxrwx 1 root root 54 2009-10-16 22:13 nm-signal-75.svg -> ../../status/48/notification-network-wireless-high.svg
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So then I did this:
root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ln -s ../../status/48/nm-signal-00.svg ./nm-signal-0.svg
root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ln -s ../../status/48/nm-signal-00.svg ./nm-signal-00.svg
root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ln -s ../../status/48/nm-signal-25.svg ./nm-signal-25.svg
root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ln -s ../../status/48/nm-signal-50.svg ./nm-signal-50.svg
root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ln -s ../../status/48/nm-signal-75.svg ./nm-signal-75.svg
root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ln -s ../../status/48/nm-signal-100.svg ./nm-signal-100.svg
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To get this:
root@fluorine:/usr/share/icons/elementaryXubuntu/apps/48# ls -alh nm-*
lrwxrwxrwx 1 root root 32 2009-10-21 09:03 nm-signal-00.svg -> ../../status/48/nm-signal-00.svg
lrwxrwxrwx 1 root root 32 2009-10-21 09:03 nm-signal-0.svg -> ../../status/48/nm-signal-00.svg
lrwxrwxrwx 1 root root 33 2009-10-21 09:03 nm-signal-100.svg -> ../../status/48/nm-signal-100.svg
lrwxrwxrwx 1 root root 32 2009-10-21 09:03 nm-signal-25.svg -> ../../status/48/nm-signal-25.svg
lrwxrwxrwx 1 root root 32 2009-10-21 09:03 nm-signal-50.svg -> ../../status/48/nm-signal-50.svg
lrwxrwxrwx 1 root root 32 2009-10-21 09:03 nm-signal-75.svg -> ../../status/48/nm-signal-75.svg
----

Now NetworkManager/nm-applet runs as expected and their is no more error messages.