Immigration at the Cobra Swamp (Suvarnabhuni airport) and at all border crossings [u]are demnding to see onward tickets on a regular basis just now, better safe than sorry I'd say.
In saying that I've never once been asked for an outward ticket before I got my non-imm' visa and work permit.
Why not buy a cheepo ticket on line for a few quid from air asia or another budget airline, to Singapore or KL, and have the confermation slip with you? If you use it fine if not it hasn't broke the bank.
Forgot to add if you want to check out what's going on whit visas or anything to do with Thailand check out www.thaivisa.com.
you'll find out anything you want on there.
Thailand is much more worried about having another El Qaeda operative found within it's borders than a bunch of backpackers. They don't mind how long you stay - within reason if you have the correct visa - that way they know you are in the country - the 30 day "pass" is not a visa and some expeats have been living here for years - even decdes by going to the border once a month. the result was that the govt had no idea who was in the country.
why people don't just get a proper tourist visa before they come is beyond me - then if they need to re-enter a couple of times there's no problem.
BTW = Backpacker is a horribly imprecise term - it defies definition.