When I used to carry an Air Force-issued BlackBerry circa 2005/2006, the Google Talk BlackBerry OS application served as a great replacement for my SMS needs. Typically the people I texted with frequently were my wife & brother, and both were heavy Google Talk users. Instead of sending SMS messages back and forth, they would just IM me via Google Talk.
Once I upgraded to a Palm Treo 700wx in late 2006, there wasn't a great mobile instant messaging application for Windows Mobile, so my mobile communication habits changed. Unless the message was urgent, we would just e-mail each other. If the message needed to be received quickly, we would SMS or place a voice call.
Upon moving to an iPhone in late 2008, I discovered another great mobile instant messaging application: BeejiveIM. It wasn't quite as fast on my iPhone 3G as the original Google Talk application was on my BlackBerry, but it got the job done.
After upgrading to an iPhone 4, though, BeejiveIM has become my go-to mobile communication app. With the new phone's more powerful hardware, BeejiveIM is lightning fast and makes sending/receiving/managing IMs on the device painless.
I only keep my Google Talk account logged into the app, but it supports all of the major protocols such as Facebook Chat, AIM, MSN, & Yahoo.
Since I normally have my Google Talk account logged in via Adium on my MacBook Pro, too, it's not uncommon that an incoming message will be delivered simultaneously to BeejiveIM and Adium. Based upon which device I respond from, subsequent incoming messages will only be delivered to that device for approximately the next 30 minutes. I consider this to be a welcome feature, as I wouldn't want duplicate messages on each device once I begin a conversation, but want the option to decide which device to reply from during future conversations.
You can learn more about BeejiveIM & find purchase information at the link below.