Hope this answers your question:
http://www.wired.com/gadgetlab/2009/...nt-kindle-att/
"Amazon Dumps Sprint for Kindle 2, Embraces AT&T"
From my understanding, there are 2 versions of the Kindle 2 - the earlier version that was exclusive to the US and with Sprint 3G and the later version that was "US and international" and with AT&T 3G.
You have the "newer" version of the Kindle 2.
Happy reading!