Do Thai mobile SIMs (I have DTAC Happy) roam in Burma by chance? What are best local options there?

I haven't been there since 1989!

You can check the service provider's web sites for the current IR arrangements.

http://www.dtac.co.th/en/prepaid/services/gointer.html

http://www.ais.co.th/roaming/en/3g-prepaid/package-rate/1/Asia/192/Myanmar

Thanks for the answer, but I already did that.

Just got news back from Burma that no roaming works there and one has to get a local card on arrival. They haven't joined the modern world just yet!

Telenor (DTAC here) supposedly has one of two recently granted licenses, and plans to build out in Burma, but there seem to be some delays/issues with the final granting of the license - maybe extra payments are still owed/expected?

They were/are hoping to have some decent coverage - population rather than geography; Myanmar is ~ 20% larger (area) than Thailand - towards the end of 2014.

http://www.globaltelecomsbusiness.com/Article/3285454/Telenor-CEO-Jon-Fredrik-Baksaas-plans-launch-in-Myanmar-later-in-2014-if-new-telecoms-law-is-put-in.html

When you arrive in Burma, International roaming doesn't work and you cannot buy a SIM card without going to the illegal market and buying an overpriced card. Around 150USD I think. The legit cards for normal prices are limited and wait listed last I heard.