On many phones, mobile instant messaging = text messaging. The OZ Instant Messenger included on Windows Mobile devices from AT&T and T-Mobile uses SMS for delivering IMs. A lot of other dumb phones also use SMS for IM. So in many cases, it's the same thing. I think text messaging and instant...

Here's a "wow" feature you can demo for almost any customer (by way of Alex D.--thanks!): Windows Live has an SMS search Beta that allows you to send a text to 95483 (WLIVE) on your phone for answers to general questions, reverse phone lookup, stock quotes, word definitions, math calculations...