Looks like Thailand is one of a few countries not covered by TomTom SatNavs as there doesn't seem to be any maps for Bangkok. Not to be left stranded and lost in and around this beautiful country I went searching for a suitable alternative mapping solution.
Garmin seems to offer a fairly detailed map of thai roads, although the routing feature doesn't seem to be as good as TomTom SatNavs. Maybe my opinion will change once I'm more familiar with Garmin's interface.