Whenever I sign out of AIM, all IMs are forwarded to my mobile phone. This is nice... but due to the large number of AIM users who aren't using the AIM client [think Trillian or iChat], it doesn't seem obvious at all when I'm on a mobile phone. This means that it now looks like I never respond to IMs. I don't like this.
Yahoo! has a more interesting way of solving this problem: instead of just a sign out option in their client, there's a sign in to [your mobile phone number] option, which is Way Cool. I like this one the best; it gives you control over whether or not you'll be reachable on your mobile, of if you'll just be offline.
Our stuff - MSN Messenger - well, it's good, but not great. Microsoft's design philosophy here seems to be insistent on drawing a distinction between "really online" and "on a mobile phone"; when you sign out of MSN Messenger, your buddy icon changes to yellow in MSN Messenger clients, which allows folks to send you an SMS. But is it easy to reply to someone that way? Yahoo and AIM both tell you that it's OK to reply to an incoming SMS, but MSN just... well... it doesn't tell you at all. It's just cryptic.
Let me see if I can get this fixed today. Finally, a mission!