I believe that Lilith as Adam’s first wife is a Jewish belief, albeit not one that is religiously binding. It amounts to about the same as Joseph being an old man when he married Mary. I can’t remember all the details of the Lilith story, however, and the serpent thing is new to me. From what I recall she was indeed an unfit wife for Adam for a number of reasons.
I believe it may have been a way for certain Jews to reconcile the two seperate accounts of the Creation of the world in Genesis, since the creation of Eve isn’t mentioned in the first account. It’s more of a “reading between the lines” belief than anything. Perhaps SSV can chime in on the matter, if she’s even familiar with the Lilith account. Not all Jews I’ve talked to are.
It’s definately not something that was “covered up” by the Church, however, as it’s never been an authoritative rendering of the Genesis account but rather a pious addendum and possible explaination of perceived inconsistancies. For all I know, it came about after the big split between the Church and normative Judaism.
Addendum: After some brief research it appears that the tale of Lilith as Adam’s first wife comes from Kabbalism, with passages from the Zohar. She is a demon, who I believe is actually mentioned in the Bible, who was later (during Medieval times, it seems) portrayed as Adam’s first wife in Kabbalistic circles. She’s more a matter of Jewish folklore, it seems, similar to how a Biblical spin on the “fair folk” was created in the Celtic Catholic cultures. Sadly my knowledge on Jewish folklore and the Zohar are not what they could be. Here’s just one link with some extensive information: