I've noticed this issue too. When I upgraded from Karmic to Lucid it automatically changed the behavior of gnome-screensaver setting to automatically lock the screen when the screensaver is active (setting "Lock screen when screensaver is active" in gnome-screensaver-preferences). When I uncheck this check-box, the issue doesn't happen anymore.
I've noticed this issue too. When I upgraded from Karmic to Lucid it automatically changed the behavior of gnome-screensaver setting to automatically lock the screen when the screensaver is active (setting "Lock screen when screensaver is active" in gnome-screensav er-preferences) . When I uncheck this check-box, the issue doesn't happen anymore.