I pulled the trigger on a Cervelo P2 today and was looking at a Shiv also (not the TT version). They guy in the store said there may be issues with racing them in non-tri events. I knew they were not UCI legal, but my assumption was that it didn't matter at a USA cycling event (at least not the levels I'm performing at). I did a quick google search and didn't find anything definative except a USA cycling statement from 2010 saying they were OK for masters, but not for any other Cat. IN the end, I went with the P2 because it seemed like a lot of bang for the buck, but would like to know the deal on the Shiv. I've got a teammate that just bought a Shiv today assuming he'd be able to use it in USA cycling TT events/stage races.
On a related note, do you think it's reasonable for a TT beginner to get proficient enough on a TT bike in 1 week to make it worth using at an event? We've got the State TT coming next weekend and some teammates have convinced me to attend, but I'm undecided if I want to try on the new bike or just ride my road bike in the EM category. The position feels really funny to me (during my 15 minute test ride) and I seriously question my abilty to make good power for an hour straight. I'm going to spend some time on it the next coulple days and see if I can get comfortable.