But I am setting it up exactly as ladyada has it, and she has no problems!
That is the problem with the internet, just because someone tries something once and has no problems doesn’t mean it is a good robust repeatable design.
It’s being powered by arduino’s 5V, too, so its probably regulated enough, no?
No - decoupling is little to do with regulation, it is to do with interference on the power line, causing interference to the device. A company I used to work for made the same mistake of not decoupling this very part. It went all through the development process and testing and it wasn’t until they had built several thousand boxes that it came to light that a significant number of IR detectors failed through lack of decoupling.