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

1-7 - switch screens
Up Arrow (without Control) - scroll current screen up
Escape - close screen (but do not part channel). This only
    works with the Start key; pressing Escape a second time
    exits from "Escape mode".
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.
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).
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.

Future plans:

J - join last channel you were invited to.
? - Show help (also the Help key by itself will do this).