New bike P2C from LBS or used?

Purchasing new from a LBS usually includes a fit, which can't be overlooked when buying a bike, new or used.  So, factor the cost of the fit into the savings from new vs. used, probably about $150-200 depending on the LBS. 

I just bought a 2008 P2C from a LBS and it included a fit (its an ongoing process) and a tune-up after 150 miles.  I bought the 5 year, no-limit service package for $70 more.

I'd say the fit and cost of a "fit" will help decide. If you've ridden road bikes awhile, or especially tribikes, and can dial in your own fit more or less, you could probably get away with Ebay. But, if you're a newbie like I was, having an LBS bike fit was critical. They dropped my stem, swapped out the aerobars (no cost), and moved the seat around - took 90 minutes total. My bike fits really well now, and I've had no problems with saddle, long rides, etc. I'd definitely go LBS again for a $$$$ bike like Cervelo if dialing in a fit is really important.

I now however, would definitely try out ebay "deals" for cheaper bikes (<$1000 on my budget) and have some fun messing around with the fit!

Go new.  Cervelo only honors the warranty on their frames to the original owner.  Additionally, the obvious suck economy is a bonus for you.  As others have stated Cervelo does not allow their dealers to deviate on price, but I would tell you to call around.  I can guarantee a dealer will "cut" you a deal below MSRP.