The UNIX shell has for ages shown to users their last login time after a successful login time. The line is e.g.: #Last login: Web Sep 23 10:57:34 EEST 2009 on tty1". This is a good security feature since it allows the user to discover if his account has been accessed at a time the user himself knows for sure he didn't log in. Command 'last' will show the last logins. Man page 'wtmp' will tell more about the feature.
At the moment there is nothing similar in a graphical environment. If somebody is using my account, I wouldn't probably notice it..
I don't know what the correct place to implement this is (GDM, Gnome, separate script?), but some part of the login process should
a) record the login to /var/log/wtmp
b) read the last login for the current user an flash it as a notification for 5-10 seconds
Additionally it would be nice if there was an graphical equivalent graphical tool to the 'last' command. However that requires more work. I guess the two things suggested above could be done with just a few lines of code - thus I made this bug report.
Even when you log in graphically, the timestamp is still recorded in /var/log/wtmp, and the "last" command still works. Still, you are right in that it would be a good security feature to somehow present this to the user on login.
gdm was completely rewritten to work with xsplash in karmic, and it isn't as feature rich as it once was. I think the priority is to get the whole login stack up to feature parity with what was there before the transfer to the new gdm first, before implementing something new, but this is still a good idea for the future.