I just returned from Bali on Thursday. Here's a word of advice for those people still planning on travelling there.
I had some problems with money changers fixing their calculators. Basically, they've switched the memory button with the equals button. You tell them what you want to change, they store their desired amount into the memory, then when you hit equals, it brings up their amount again.
If you haven't already worked the amount out in your head, do something simple like 1 + 2 and you'll see if they're being tricky or not. Then it's just a matter of asking for your money back, which they will generally do without hesitating as they know they're in the wrong.
I only encountered this with places that weren't exclusively money changers, ie were different shops that also changed money.
Be on the lookout! I really support any of you that are still travelling to Bali, as I said I returned 2 days before the blasts, but I'd book another trip in a heartbeat if I had the money.
Please keep travelling to Bali, they really need our support.