Thanks for the reply. I think I've actually traced both the logout and lock-up problems to the GNOME panels. For example, I found another thing that will lock-up the system: if I drag one of the panels to another part of the screen, it also locks-up. Actually, it just locks-up the panels--other open programs still respond.
Plus, I've tried killing the session after such a lock-up happens and then re-login, only to get a message that "GNOME found another panel running", meaning that the locked-up panel never died when I kill the session. Stubborn things.
So I'm going to look around for panel bugs, but if anybody knows up front what the problem is, I'd love to hear!
(BTW -- My panels aren't loaded up with very much; I only have GAIM, power applet, NetworkManager, the mixer, and the calendar running.)