irssi 0.8.10 has problems with cursor keys + modifiers from xterm. 0.8.11 fixes this for ctrl+cursor keys, but not for alt+cursor keys, meaning you can't use alt+left/right to switch windows. See upstream bug at: http://bugs.irssi.org/?do=details&task_id=496
For reference - http://invisible-island.net/xterm/xterm.log.html#xterm_167
James, thank you for the little patch fix. It's in irssi-0.8.11-4.fc8. I will push up upstream to commit this little change to their svn.