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 messaging as equally intrusive though. They're both messages that contain text and they both arrive instantly on your device. You can set the notification options on each so that maybe one does not make the phone play a notification sound, but they seem to be pretty much the same.
I do love the new Windows Live Messenger though. If I have time to kill waiting for some one in the car, it's pretty entertaining to be able to exchange voice clips with others... maybe send a self portrait JPG too. It really needs to implement the Microsoft Portrait video chat features though and make better use of the front facing cameras on all of these Windows Mobile phones.