Hold down Start, *or* press and release Escape, and:

1-7 - switch screens
Up Arrow (without Control) - scroll current screen up
Tab - switch to last displayed screen
Left/Right (without Control) - previous/next screen.
A - switch to active screen. [TODO: if no active screen, move
    left one 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.
L - list all screens (number and name).
W - in query, /whois the user. in PM, last user who PMed.
    in a channel or [server], no effect.
N - in channel screen, do a /names #channel. in [server], last
    channel with traffic. no effect elsewhere.
P - in a query screen, do a /ping <nick>. in [private], last
    nick who PMed. no effect elsewhere.
T - /topic #channel
E - show full editbox (instead of screen).
X - close screen (but do not part channel).
Ctrl-X - in a channel screen, /part the channel and close the
    screen. no effect elsewhere.
Ctrl-D - disconnect from server (without sending a /quit). This
    *doesn't do anything useful* for users, it only exists as a
    debugging aid (so I can test the automatic server ping code).
    This will likely be removed before the first beta.

If you press Escape by accident, pressing it again gets you out of
"start key toggle" mode.

Future plans:

1200XL F1-F4: without Shift, switch to screens 1-4. With
    shift, F1-F3 = screend 5-7 and F4 is same as Start+A.
Help - same as Start+A.
J - join last channel you were invited to.
? - Show help (also the Help key by itself will do this).