The "Feds" (i.e. The IRS) Don't care, nor can they enforce whether or not you comply with the state vehicle registration system. It's not their place to enforce vehicle registration with the state and they won't spend the time to follow up with all 50 states registration system. The credit depended on you buying the vehicle, which was certified by the manufacturer, in the year the tax credit was applicable.
Register if you want. Worst case scenario you could get a ticket by a state, county or city law enforcement official. But if anyone sees a black & white IRS car with lights flashing, let me know!
Here is the Internal Revenue Bulletin 2009-26 which states the criteria for which must be met for the credit to be filed and I can't see where it says it must be registered or titled. Someone take a look and see if you see anything different.
http://www.irs.gov/irb/2009-26_IRB/ar07.html