However leaving this as a site issue (albeit not a particularly pressing one) is not unreasonable. I would think that eventually there should be some kind of filter to protect against this.

Well, potentially harmful HTML is already stripped from posts. I've mentioned this particular bit about Word's junk comments to Andrew, and he said it's on his list to look at.

It's not an issue in any browser other than IE because only IE supports conditional comments--to every other browser they're just regular comments and are completely ignored. Still, kinda funny that a Microsoft browser can't handle their own special HTML.