It's bad enough now; it could be even worse. They could respond on port 443, too, with a legitimate-seeming certificate -- they're *Verisign*, the leading certficate authority.
In the security world, we call this a man- (or monkey-)in-the-middle
attack, for which the standard defense is crypto. But that doesn't
work well when your trusted third party is part of the threat model...
--Steve Bellovin, http://www.research.att.com/~smb
