Hi shadow, Ive just come back from Thailand, took $3100AUD over, which consisted of $100 changed into baht here, $1500 in cash and $1500 in t/cs. While the exchange rate to change money into baht here is indeed always much lower than you'll get in Thailand, it does come in handy for little things you may need, before you get a chance to change money there. For example, when we were in transit at BKK airport, it bought food and drinks, and you may need it for a taxi or transfer to get to your hotel. I think I changed $100 and only got about 25bt here. In Phuket and Bangkok the rate will be much higher, and as Alison says, the change rate for t/cs is always higher by about 1bt. The dollar keeps going up and up, and you'll prob get about 29bt or more now which is great! If you feel safe enough about carrying cash and where you are staying has a reliable room safe or reception safe, then take cash. I did find t/cs were the cheapest alternative to not taking cash, bought them at the PO and no fee to buy them, but about 30bt fee each to cash them in Thailand, plus you will need to take your passport with when you go to change them. Also, if you are with a credit union, another cheap alternative to accessing cash is doing an atm withdrawl on your credit card, I did only one in Bangkok and it only cost me a $4.00 fee, which I thought was OK.
Anyway, there are lots of ways to take money over, there's also the wizard card Karin talks about, and there's a travelex card, and also you can use your atm card over there too, although I think using an atm card does have higher fees than doing a credit card withdrawal.

Kitty