If you're listening to XM for the most part, you won't run into a problem with the data limit, especially if you use WiFi from time to time.
I have a co-worker who streams Pandora all day long thru his phone, thru 3G, not WiFi, and our company has never hit the data cap limit.
We've had 5 Mez's on our corporate account since they came out in October, all of us use them heavily for streaming music. Our boss has not complained once.