I use this pattern religiously (I've learned some interesting things about who gives my email address to whom), and the problem has become a lot less prevalent over the last year or two, but it's still an issue in some places. Until recently, even Facebook and Dice.com wouldn't accept a '+', and while bn.com would accept it, the Barnes and Noble e-reader iPhone application wouldn't (which they've since fixed; I'm still absolutely floored by the amazing customer service experience I had with B&N as a result, compared to what I expected).

I've reached the point where, if your email validation is that badly broken, I'm probably not going to trust you with anything more important than that. It's made for an excellent trust litmus test.