I use this on my treo and it is pretty bad-ass becuase it supports multiple accounts on multiple systems (blackberry,palm,winmobile,j2me,brew,symbian phone on yahoo, msn,aol,icq,jabber,googletalk,lotusnotes,LCS,and SIP)
The cool thing is that their server works as a proxy. you contact the server via your cell phone, the server logs into your IM account. So what's the benefit? You will always look like you are online and available. SO what happens when you take your phone out of data mode to make calls? it disconnects your phone from the server but keeps your status shown as online. When someone sends you a message, the service sends you a text message. You can reply back with a text which the server re-writes to your IM account. You do need an unlimited messaging plan on your phone or this will cost you plenty.
http://causerie.mantragroup.com/index.php