I had a quick chat with the folks from Pinger Phone while we were at Macworld 2009. I hadn't heard of it previously, and it turned out to be exactly what I needed to bring together my various forms of communication. It's an iPhone/iPod touch app that provides a contact list combining email, phone, SMS and IM info for all of your contacts. One of the neatest features of the app is its ability to handle text-to-IM chats, which I find useful because I have terrible AT&T coverage around my house; it lets me carry on a text-message conversation using just my wi-fi connection.
Pinger Phone lets you stay online with various IM services, and can notify you of incoming messages as long as the app is open. It also supports (requires, actually) horizontal keyboard mode in text and IM chats, and interfaces well with the phone and email capabilities of the iPhone. The "Contacts" view allows for as-you-type searching of your consolidated contact list (very speedy). iPod touch users can enter any mobile phone number to be notified of incoming messages from a text-to-IM chat.