I just finished my first build (and sold it, actually - figured I would build one to learn the ropes before I build one for me to keep. Made $50 on the deal, too, so I can't complain.)
I talked to a local police chief who retired last year from the force but still works in law enforcement. He is adamant that anything under 50cc is legal with no registration, inspection, or title. Not even a driver's license is required. Apparently they had a couple people in town who used to ride them (Whizzers, I think). One rode the bike because he had lost his license to a DUI. The other just generally cruised around being an idiot. I guess they have him a hard time about the stupid stuff he did, but didn't see anything wrong with him simply riding the bike.
He copied me out a few pages from the law books which lead us around the same old circle we've already been around in the first 21 pages of this thread. He said not to call Penndot - first, they will give you a different answer every time, and second, they always have a feeling that you're trying to sneak something by them, so they tend to just say "no" if they're not sure.
He said that the police department actually had a letter on file from the Attorney General's office clarifying what was required of motorized pedalcycles (absolutely nothing). They had written for clarification when they had a problem with a local store (Country Junction - the World's Largest General Store {until it burned down}) selling little electric scooters to kids and telling them that they were perfectly legal to ride on the road. The letter from the Attorney General stated that the only vehicle that could be ridden without license, title, or registration was a motorized pedalcycle. He wasn't sure if they still had a copy of the letter on file, though. Maybe someone should write/call the Attorney General's office for clarification.
He was totally convinced, even adamant, that these bikes (under 50cc...cough..cough) are perfectly legal. I've posted a pdf of the documents that he has given me. As I get more from him, I will post them here as well.
EDIT: I can't post anything over 19.2 kb here. If I can find another place to host it, I'll post a link. Otherwise, email me at foeller32(at)yahoo(dot)com and I'll email a copy back to you.
Good Luck!