A number of large vendors like Barracuda and Tipping Point do this. A friend of mine one time found that they were unable to log in to pay their TV bill a few days after setting up a relay node. After he made enough noise about this, their engineers investigated, and sure enough, Tipping Point flags a relay node as suspicious for no good reason, and the IPS was blocking his traffic.

On a side note, companies really don't like to deal with people who can't access their site. Being told over and over "It's your ISP", even when on a conference call with said ISP (and nevermind how difficult it was to get that set up with someone who understands the problem) was infuriating.