I ordered the iphonetrip.com 3G SIM for traveling in Italy next week.
I was going back and forth between them and Italy carriers, but the big decision maker was I did NOT have to unlock my phone to use their SIM.
During my stay, I will be traveling around Venice, Florence and Rome and hoping this service works the entire time... I had them send me an email confirming 3G coverage along my journey in those spots.
I'll make notes and post the findings, but from what they claim, I am 100% covered