This is a very common technique know as
phishing (
http://en.wikipedia.org/wiki/Phishing). It is not a Comcast issue per se it is an issue with underlying protocols of Internet email.
Anyone can send an email and make it look like it was sent by ANYONE. You should never trust the From: line of an email.
With any email you should be suspicious of links or any attachments and be sure it really came from a trusted source.
Looks like you did the right thing....