Occasionally, I seem to run into oldgrown bugs, which haven’t been resolved in years now. Since I started to use irssi within screen again, I ran into the infamous “ALT+[0-9] is not passed directly via xterm to irssi” problem again – I do remember having that some years back too when trying to switch channels in irssi.
Resolving that issue is rather simple, if you don’t care about possible side effects within xterm itsself.
$ vim ~/.Xresources XTerm*eightBitInput: false XTerm*metaSendsEscape: true
To activate it for the current X session, run
$ xrdb $HOME/.Xresources
and then start xterm again, connect to your box, attach the screen, try to switch between irssi channels with ATL+[0-9].
Works like a charm!
Update 2012-12-16: Putting those options directly into /etc/X11/app-detaults/XTerm(-color) makes them the default systemwide.