I received word today that the UK Foreign and Commonwealth Office has updated its travel advice for Thailand regarding the floods. In a nutshell, it is advising against all but essential travel to the affected provinces and also to the city of Bangkok, as flood waters are expected there up until 31 October.
Note that they state Suvarnabhumi Airport is still open and unaffected by flooding at this stage. Their advice not to travel to Bangkok does not include transit through the airport; if you are transiting and flying out to Phuket or Samui then all is fine currently.
Please be aware that sometimes such government bureaus take an extra-cautious line when advising to travel, but generally their reports are factually correct. Use their advice together with the advice of people and other travellers on the ground in Thailand (on this forum and others).
One point to be aware of is that some insurers invalidate your insurance if you travel to an area that the FCO (or international equivalent) is advising against and have to make a claim in such an area.
The updated advice is here:
Thailand travel advice