Just noticed this thread. i would second the recommendations for the Blue Elephant and Celadon (at the Sukothai). It is a tough call between these 2 places for the best Thai food I have ever eaten.

For Sunday Brunch the Four Seasons does a great spread, and martinis (a variety of types) are included. I also like the Cafe Conrad for Sunday Brunch.

La Dalat (the original branch) has great vietnamese food and I thought the prices were very reasonable. For a different experience the food court at MBK is also worth a try.

The Banyan Tree does an excellent Dim Sum lunch at their chinese restaurant whose name temporarily escapes me. It is supposed to be a buffet but is served at your table, and the restaurant has awesome views (being at the top of the building, just underneath Vertigo).

For sushi/japanese I have seen the recommendations for Zen, I however much prefer Fuji which has a better menu and tastier food IMHO at very reasonable prices. On the subject of local fast food, no trip to BKK is complete without at least trying the Suki Yaki at MK. It isnt the best suki in the world, but is very popular with the locals (as the queues will show).

Last but not least Neils Tavern on Ruam Rudee serves the best steaks (and seafood) in BKK. Ruam Rudee also has many other high quality restuarants at reasonable prices.