Announcement: Plus signs in e-mail addresses ARE VALID! I can't tell you how many time I went to a blog, a forum, or a (poorly design web...)store and my e-mails didn't pass validation. On some blogs I even get ASP.NET exceptions.
Plus signs in e-mails are commonly used for sub-addressing. When you have an e-mail like email@example.com and you don't want to give that to a webstore you can simply hand out firstname.lastname@example.org or help+Amazon@gmail.com or whatever and you're still good.
So PLEASE... if you wrote a buggy application or script that disallows '+' in e-mail addresses, fix your bugs.