Kindle 3 (Keyboard), yes. But it is restricted to what it is originally intended for - namely usage from the device. "You agree you will use the wireless connectivity provided by Amazon only in connection with Services Amazon provides for the Device.
You may not use the wireless connectivity for any other purpose."*
Kindle Touch and Paperwhite: no. You can browse Wikipedia only, and shop in the Amazon store.
(*) See also:
this.
PS: There have also been reports that the data download limit is/may be capped to 50 MB when roaming. Search this forum for more info.