Having just talked to a traveller at the guesthouse I'm staying at, she arrived late last night and crashed in one of the dorms (8-person dorm). While the guesthouse offers a locker, she opted not to use it and woke this morning to find her daypack gone. In it was her brand new camera, iPod, all her paperwork (passport, credit cards, travellers cheques etc). A bit of a bum start on the first night of what was supposed to be a two-month-long SE Asia trip. She's (understandly) pretty cut up about the whole thing, and given four of the people in the dorm have already upped and checked out, nobody will know what really happened to her gear.
Moral of the story, if your guesthouse has a locker service -- USE IT. And don't assume that all travellers are totally honest -- unfortunately too many opt to prolong their travels by ripping off other travellers.
Yeah -- I didn't mention the name of the place -- nor the country for that matter -- as I think it's general good advice -- watch your stuff, and if there's a safe or a locker -- use it!