Comcast has done this on a large percentage (perhaps all) of their customers' equipment as a preventative measure. It's not because of anything you've done.
Being one of if not the largest ISP in the country, they get a lot of spammers using their service.
This is a good thing, IMO. Comcast will likely have to spend millions on support calls from confused customers, but I think they're doing the right thing. I don't agree with many of their practices, but I think this one's good.
I agree that it is a good thing overall, but if Comcast mass-mailed a letter that said:
"We’ve taken this action because we may have detected virus-like activity from your modem or received reports from other email providers that mail from your modem generated complaints from their users. and that really wasn't the case at all (that it is just a mass mailing) then I think that is really awful. Deceitful. They should have just yelled "Achtung! No more unauth'd SMTP! Switch ports! Now!"
I say awful because that wording could reasonably send lots of people into a panic. I hope this wasn't the case...that it was actually based on specifics. Comcast sucks.