There’s now a new Steam app for mobile devices that’s currently in limited beta. The best part about the new app is that it allows you to chat on Steam Friends, without using a Steam client, which means a Pidgin/Adium plugin is more likely to be on its way. The second best part is that you can be logged into the mobile client at the same time as being logged into the desktop, so signing into this mythical Pidgin plugin wouldn’t sign you out on your desktop machine.
Let me know if you’re in the beta at the moment (ie, have you managed to sign into Steam from your mobile) as I’d be keen to hear your experiences with it.
Edit: The Steam plugin for Pidgin has been connecting to the mobile API for a while now; Steam no longer needs to be running to be able to chat with your friends