this works fine for me (mIRC v6.12)

Code:
alias f8 {
  if ($away) {
    away
    tnick %away.nick
    unset %away.reason
  }
  else {
    set %away.nick $me
    set %away.reason $$?="enter away reason"
    away $!
    tnick $me $+ _away
  }
}

Go ahead, jump. 100,000 lemmings can't be wrong.