Well I`m back now.
Re Google Voice, I have no idea - never used it.
My AT & T sim worked ok - well it did after they fixed the 3G antenna in the area - that was a little confusing at first till I called them.
I activated my sim online about 4 days before I went, used the hotel address when registering - I think you get cheaper calls in your `cell` area, anyway, knew I wouldnt be making many calls, so added $25 to my account when registered and then when I was in the States I called AT&T customer services (611) and had $20 of that transferred to be used as 3G, you have to get them to activate it for you too.
One thing to note is that I`m on Voda over here (and with my old sim card it connected to AT&T anyway), and I get HSDPA data connection, but I dont think they have it there as I only ever got 3G, so not quite as fast.
Enjoy.