FWIW here's my experience as far as 4 wheels is concerned. I drove through India and reached Calcutta in Jan 08.
I would have liked to drive from India into Burma, but didn't even bother trying as I had been told it was impossible. So I stuck the car in a shipping container in Calcutta and jetted off to Burma with a backpack for a couple of weeks before rejoining the car in KL.
Imagine my surprise when I met Thierry and Coralie in Inle, with their French-registered Landrover. They had driven in from Thailand, through the Mae Sai border.
Overland from Mandalay -->Imphal ... - Thorn Tree Travel Forum - Lonely Planet
I tried to find out how they had managed to negotiate the bureaucracy to get into Burma, but they wouldn't tell me - didn't want to open up the floodgates (as if!) I think, or rather I know, that Thierry was very good at talking to people and getting what he wanted without directly asking for it.
But you will be interested to know that even Thierry, having enlisted the help of the French Ambassador to Burma to speak directly to his Indian counterpart, was not able to get a permit from Rangoon to travel through the North Eastern Region of India. This was categorically NOT the Burmese junta refusing him, but the Indians. I am told that this was nothing to do with visa issues, nor where the permit was applied for, but due to a then-imminent crackdown against separatist rebels in the NER, which the Indian government did not want foreigners to witness. So Thierry and Corali had to return to Thailand through Mae Sai.
By all means go to Burma, but I wouldn't get too worried about not taking your own vehicle. If nothing else, you will have a lot of difficulty getting decent fuel for it - Thierry and Corali had to threaten a diplomatic incident every time they wanted diesel from the government filling stations. I remember one time when they ended up buying 5 gallons of government diesel, at the black market price, only to see the government filling station people promptly using their money to buy 5 gallons of black-market diesel from outside the station and pour it back into the government diesel tank! We heard many stories of people's engines being ruined with black-market fuel; government fuel is rationed in Burma, and can only be bought with ration books issued to vehicles registered in the same division as the one they are buying fuel in. Getting a foreign-registered vehicle into Burma is obviously such an unusual phenomenon that nobody has thought how to address getting fuel for such vehicles!
I was also told in Calcutta that the border between the NER of India and Burma IS open, but only for Burmese/Indian citizens ie no 3rd country nationals.