Software offers the security necessary for mobile workers, and is compatible with BlackBerry, iPhone, and Windows Mobile smartphones.
Device Index IM
Even without its workarounds for the iPhone's push data limitations, BeejiveIM is a pretty good multi-network IM client.
SHAPE's IM+ provides a way to receive instant messages when the iPhone's busy doing something else, but it does so in a barebones setting.
Don't panic when someone sends you an SMS, instant or e-mail message that seems to be written in a foreign language. This list will help you decipher the lingo so you'll know you asp from your EMA.