T-Mobile's SMS servers automatically convert any incoming text that is greater than 160 characters into an MMS. They don't simply chop the text messages into multiple SMS like some others carriers do.
Also, e-mail formatted as HTML may also (not always) be interpreted as an MMS.
Once T-mobile sends the text out as MMS, the Treo 600 will show it in the MMS application. That's normal behavior. Most T-Mobile subscribers have a "bucket" of free MMS, so you won't get charged anything extra for a few MMS arriving by mistake. But if a lot of SMS get transformed to MMS, then you'll see a bunch more little charges for each MMS.
