I was under the impression that XM also transmitted GPS as a part of their feed. Of course I might be completely off-base here. But I thought frodo's new app used GPS extracted from XM (i only skimmed the site so i may have mis-interpreted).
XM radio only has two geosynchronous satellites. To get any sort of GPS reading, you typically need at least 3 satellites (4 to get altitude).. and even then your accuracy may not get over 200ft. To get the 30ft or better accuracy you need to pull in 6 or more satellites, which most GPS receivers can (I think most can do up to 12 satellites).