I'm meeting some friends in Thailand next month, and am wondering if anyone has transportation tips-- sources for cheap flights or suggestions for interesting bus routes with stops in interesting places for the return journey.

Are you in Kunming now?

You can take an overnight train to Nanning (Guangzi Zhuang Autonomous Region) and then take a bus from there down to Hanoi (takes about 6-8 hours). Apparently, there are really cheap flights (less than $20US!) from Hanoi to Bangkok on Thai Air. You will have to pay for a Vietnamese Visa, though. It's 350RMB in Kunming (the Vietnamese consulate is one bus stop before the main train station) and it takes 2-3 days to process the visa (or you can pay extra to get it faster).

Oh, the train costs 181RMB for the bottom bunk of a hard sleeper (slightly less for the middle or top) and the bus is about 135RMB. I think you have to wait til you get to Nanning to buy the bus ticket.