Well, as far as I've been able to find out, the timeline for this smear piece is something like this:
Late December 2006: email starts circulating (see
Snopes.com article)
Jan. 17, 2007: Washington Post publishes
article saying that Obama will formally announce his intention to run for President in early February. This is the earliest MSM story to that effect I've been able to find.
Jan 17, 2007: Insight Magazine publishes
article repeating most of the claims in the email, makes unverifiable claim that the source is someone in the Clinton campaign.
Jan 22, 2007: CNN
debunks "madrassa" claims.
Jan 23, 2007: Obama campaign
responds, citing CNN's investigation.
Jan 29, 2007: New York Times
rains on wingnuts' parade
Jan 31, 2007: Insight Magazine publishes
hair-splitting non-denial denial, emits customary whining about "liberal media".
This is all close enough together that it doesn't point clearly to any one source, however, it also doesn't foreclose the possibility that some writer for Insight received that email (which seems to have been first out of the gate), decided to make it into a story and tried to sex it up by attributing it to another hated Democrat. Whether Obama's impending announcement, which was probably expected in political circles before the W.P. article, was a stimulus is impossible to tell; I just find the temporal coincidence potentially interesting.
The original source of the email, which is probably irretrievably lost, is what we would want to know.