I haven't had many problems getting prepaid GSM SIMs. For Cyprus I bought an official pack, for Italy ("Wind Mega No-Limit") it was trickier, but I used somebody in Italy who charges €5 to register them locally. For the US I have bought T-Mobile prepaid SIMs off ebay, for about $15 you get a card with $30 credit with a 4-6 week expiry. Annoyingly I bought one back in Nov and forgot to load another $10 which would have extended the other $30 until my next trip (SF/SJ, next week)

Gareth