Hi Jens!
I found for my last spottingtrip March 2009 via planespottinghotels.com this hotel "Queen's Garden Resort"
Link: http://www.plane-spotting-hotels.com/Bangkok.htm or Prompong J.' last post. You can spotting on the roof area (sunrise to 13:00 pm).
I'm going to next next year in March to Hong Kong, Kuala Lumpur, Singapore and Bangkok for holiday/planespotting! Sorry for my english!
regards
F. Barella