IM or SMS. Keep getting that question, and getting stuck. Having used both a ton, I can say with higher confidene than others I've talked to that IM on a mobile device sucks, when the person on the other end it a desktop user. A friend of mine, upon noticing that the user shows up as Mobile in the buddy list begins "..sending a rapid series of open ended questions" like:

- what applications are likely to be included in the long term wireless device roadmap?
- What's the role of edge network technology in wireless networks?
- will IETF and 3GPP and 3GPP2 and OHG and IEEE and ITA create a combined standards group..how woud that function alongside OMA and other similar groups?

etc. That has not stopped the enthusiasm and press releases on the subject. Here's the thing, SMS is really mobile email; just look at the attributes. It is a store/forward type of message system that does not require presence. IM on the other hand is a real-time system that depends on presence for it's power. Wireless wide area network coverage will always be a bit spotty. IM use depends on a stable, reliable connection for it's real-time use. For stationary "mobile" users this will be fine. especially when the new PDA/Smartphone are more common (read: "cheaper and better").