Yes, I've noticed something similar, although I do not let it store my ssh password on the login keychain. First time I ssh after a reboot, it asks me, I enter it.

Pre-Lion, it was then good until the next reboot. In Lion, it occasionally asks me to re-enter it. As you noticed, this seems to correlate with coming back from sleep or screen lock. It doesn't seem to happen every time, though, so I'm not sure what is going on.

According to ps, it is using the -l option to ssh-agent. The man page and the usage message of ssh-agent do not admit the existence of such an option. Perhaps that has something to do with it.