I heard from someone booking a ticket on Thai AirAsia that the departure tax will not be paid separately as of February 1, 2007, but rather included in the price of the ticket. I went to their website and made a dummy booking to confirm this.
Up to 31st Jan 2007 Thailand Airport Tax for International destinations is payable to the Airport Authorities in Thailand upon departure.
International departures from 01st Feb 2007 onwards. the Airport Tax will be included in the Total Amount Due payable when a booking is made via any of our booking channels. Bookings made as of 4th September 2006 for 01 Feb 2007 onward travel, the Airport Tax has been included in the Total Amount Due. There are no changes for the payment and collection for domestic departures as the Airport Tax has always been included in the Total Amount Due.
I'm guessing TG and all airlines will be doing the same, but don't have confirmation of this. Also I believe February 1 is the date of the increase in the departure tax from 500 THB to 700 THB.