For those of you who haven't seen this before, it's a trick supported by some email systems that allows you to receive mail to your regular address, but also allows you to figure out where that mail came from. So if, for example, I'd registered on somesite.com using waider+somesite as the mailbox part of my address, and then I subsequently received spam from another source to waider+somesite, I'd have a good indication that somesite.com leaked my info to spammers. Of course, I'm mildly surprised that no spammers appear to try to fake this.
 This is old-school web stuff: once upon a time, you represented spaces in a data submitted to a web form by replacing them with plus-signs. This has various technical explanations, but ultimately it boils down to laziness on the part of the guys who developed the system. As soon as people figured out that, hey, this made it difficult to enter actual plus signs, a new means of submitting data was decided upon, but the old pluses-to-spaces thing remains in place for backward compatibility with, like, the 500 people who were using the web before it was determined that this was a bad idea. And this bad idea still trips people up, even big-name people who really should know better.