You could probably do your thing with ad-hoc distribution on the first 100 iPads. (Actually I think that probably violates the agreement. Don't do that.)

For the iPad in particular, there has to be a way for such apps to get on, or you're never going to see anything resembling enterprise adoption, so i'd expect this to change.

For the good of the discussion, you can only get an iPhone enterprise developer certificate if you have 500+ people in your company and a Dunn & Bradstreet number. Then getting a certificate is fairly easy. This allows you to, as others have said, distribute iPhone apps over a tether.

Yesterday in iPhone 4.0, Apple announced they were now enabling over-the-air install of in-house apps. My current understanding is that this relates to opening specific APIs relating to OTA apps. (full disclosure: I work for a company called Ondeego that delivers in-house iPhone app distribution as a service.)