My boyfriend and I are flying out to Thailand early in December and are looking to spend about 3 months travelling through SE Asia. Does anyone know if we will have problems finding accomodation over the christmas period? Would it be wise to book something in advance? Although we have estimated roughly the time we will spend in each country I know that this is likely to change and don't really want to be tied down to arriving somewhere on a set day if we can help it.
To the best of my knowledge, the answer is yes, this is the high season and it will be somewhat more difficult in certain prime areas. However, places like Bangkok have so many hostels/hotels/guesthouses that you will have few problems. Remember TET falls in Jan '09 and you should be careful when trying to negotiate Vietnam at that time (in other words, book ahead).
Things are definitely busier around the holidays in Thailand, but not too bad. I had no problem finding places to stay in Bangkok (Khao San) in the days leading up to Christmas, nor when we returned shortly after new years. The couple days on either side of Christmas it was hard to find a place in Phuket (and no doubt other popular spots like Samui, and Phi Phi), however, between Christmas and New Year's our group of 6 had no problem showing up on Lanta and having many many options to choose from, ditto with Tao right after New Years. Trying to find accommodation around Koh Phagnan for the New Year's party was a nightmare. If that is a priority, then book those few days in advance.
So I think as long as you are ok with going with the flow and prefer to stay away from the few most popular Islands, then you'll be fine and there probably is little need to book. But if you want to spend Christmas Day and New Year's Eve right in the thick of things, then book those days in advance.