One that Apple states is that you need a developer certificate to run an application on an iPhone. The documentations goes so far as to tell you how to create one, but there is currently no way to upload one to ADC. You can't upload one until you've been accepted as an iPhone developer, and so far, I haven't heard of anybody who's been notified that they've been accepted. It's possible that some people have, but everyone I've talked to is still anxiously waiting.
But more important than the developer certificate is the fact that Apple hasn't released the 2.0 beta version of the iPhone software which is needed to run third party apps. In other words, even if we had our certificates, we couldn't actually do anything with them at the moment because our phones need to be updated to run these apps. Rumor says the beta version is going to be released very soon, but there's no official word out of Apple about it yet.
Unfortunately, the bottom line right now is that we have to be patient.