August 2013 update
 

Now I would say that I am getting 3G over 95% of the time on the above MVNOs.
In fact, if I choose 2G it won't go or will only remain on it for a few minutes before going back to 3G.

Does your phone support 1700mhz HSPA or is this part of their refarming?

This is a GSM 850/900/1800/1900 Pantech [I have different models].
It shows network names and 2G & 3G. So I get a list of 4 when all show here in Southern California. Definitely refarmed.

I need to take a census and start a thread on which international SIM cards work on AT&T 3G. Piranha does. Don't know which frequency though.