The evidence is pretty compelling to me:

1. Lampkin says he knew Joe Adama, who taught him everything he knew. An Anakin/Darth Vader revelation if ever there was one.
2. Eye contact is very important on the show, and there's a blatant closeup eye-contact scene between Lampkin and Apollo.
3. Leoben is said to mix truth and lies, and the statement "Adama is a Cylon" does in itself mix truth and lies if you take this interpretation.
4. It is definitely a holy shit moment.
5. There's a prominent appearance of Joseph Adama's historical record at the end of S3. The unearthing of his law books would seem to serve some larger purpose than refocusing Apollo's career, instead reminding us of the character who's going to be appearing himself soon.
6. It offers an entirely new twist on human/cylon half-breed.
7. The implied similarities that would permit transfer of a human consciousness/soul/whatever to a Cylon body parallel the ongoing "we're more alike than you know" storylines, e.g.: shared visions.