FWIW, I unlocked my ATT iphone 3GS and swapped in a straighttalk sim card and have no problems at all. (Note that straighttalk sells a sim card that can be used on a locked ATT iphone, as well as one that works on an unlocked iphone).
I have no idea about the "block" on international texts, but I would think that imessaging between the phones would work even if there is such a block.