The menu is still there, just its "decal" is screwed up. If you left-click the "Me menu" button with your name on it and mouse right, the "Indicator Applet Session" menu appears properly.
There was a major 10.04 UI patch bundle that went out in the last day or two, I've not seen this buglette reappear since. Just use the normal System ➤ Administration ➤ Update Manager and this may go away.
update: Further research shows this bug has been bouncing around Ubuntu/Gnome for nearly a year with a couple dozen bugs filed against it, poor reproducibility, and not-so-horrible effect. No one can decide whether it is the window compositor, gnome-panel, or the indicator applet session. As the "central" bug report notes
"...Please don't confirm the bug it is
already confirmed. Please don't attach
further screenshots - we all know how
the problem looks..."
so you might just want to sign on as a "subscriber" to the bug report to be notified if something changes; I wouldn't hold my breath.