Foreigners DO need a work permit to work in Thailand.
All foreigners in Thailand needs to have a work permit if they want to work here. Even foreign Bar-girls and Lady-boys need a work permit!
Working without a proper work permit is illegal and if caught the foreigner will be jailed, fined and deported from Thailand. So my advise to all you foreigners working illegal in Thailand:
Be aware! Even you only work for pleasure and not for profit, you still need a work permit!