IM+ 3.1 with Push made it through Apple Review
Finally it made it through. Agile is on the way also 
With widest support of IM networks including Skype™ chat and even Twitter and coming Push there's absolutely no need to look elsewhere.
Get the most reliable and feature-rich instant messenger.
• New! Twitter
• New! Skype™ Chat
• Facebook® Chat
• Yahoo®
• AIM® / iChat / MobileMe®
• MSN® / Windows Live®
• ICQ®
• Google™ Talk
• MySpaceIM
• Jabber®
Finally BeejiveIM 3.0 with Push for iPhone OS 3.0
You could only use one application at a time in iPhone just because apple wanted to save its crappy battery life. After 2 years now they put push support in the OS and now today Beejive comes with Push. What push means? It means you will receive messages or stay online even though you press the home button and doing something else.
Beejive is now using their own licencing and blacklisting service so bad news for the freeloaders. Second best is the IM+ in my opinion and then the best one for mobile for long years Agile Messenger. They are both coming with PUSH support in couple of day be patient