Does anyone know if a 2G SIM card will work OK in a network unlocked 3G phone?

I've taken 2g phone sim cards and put them in 3g phones before without a problem. Don't know if it'll work the other way.

As long as you don't mix up systems it'll work!

Two main systems are the CDMA network and the GSM network. Both have their 3G offerings (CDMA2000 EV-DO and UMTS respectively)...

Since these two systems are technically pretty different, very few phones can handle both. There are some, but I doubt they are 3G capable...

For a definitive answer it would help to know the make and model number of the phone and the service provider for the SIM.

For Thailand if the phone supports GSM900 then a local SIM would work fine.

My phone supports GSM900/1800/1900 and WCDMA2000 (3G). I use a 1-2-Call pre-paid SIM here in Thailand successfully, and a T-Mobile pre-paid SIM in the U.S.A. successfully, and can ride on Vodafone's 3G network in Japan.

are there any plans for 3g to become available in thailand or are they going to wait for the 4g offering with the faster mobile data speeds

Two main systems are the CDMA network and the GSM network. Both have their 3G offerings (CDMA2000 EV-DO and UMTS respectively)...

Nokia N series handles both.