Some IM apps use SMS -- others use data. That's why you'd need to clarify regarding the IM app you intend to use.
SMS and MMS aren't considered data (though MMS on most BB's requires a BB data plan).
Originally Posted by Kagasumi
Sprint doesn't offer the 8310, which sucks. :( They do offer a 8830, but that does not use EDGE.
It doesn't suck. The technologies are incompatible. They'll never offer the 8310. Similarly, there's no need for an EDGE 8830 as there are several GSM counterparts in the 88xx series.
Here are the closest equivalents:
at&t - Sprint
8800/8820 - 8830
8300/8310 - 8330
Again, always pick your carrier first.