Hold down Start and:

1-7 - switch screens
Up Arrow (without Control) - scroll current screen up
Escape - close screen (but do not part channel)
Tab - switch to last displayed screen
Left/Right (without Control) - previous/next screen.
A - switch to active screen
S - show status (hide edit box)
Q - create screen for last PM (in [private]) or last channel
    message (in [server]). Does nothing in screens 2-7.

Future plans:

? - Show help (also the Help key by itself will do this).