>>If not monitored, can easily become the most relevant term on your site.
If they were truly counted as a word, then you would think that it would be something that was indexed. A quick search for * on AV and Google certainly didn't return enough pages to suggest that the tag is indexed as a word. The only pages that show up are either articles that talk about the tag, sor pages that are coded poorly and the tag is being displayed by the browser.
On the other hand, I'm a firm believer in cleaner code=better rankings, so I'm sure it can, if overused, have a negative effect. I just don't think it really is a keyword density issue. More of a file size/download time issue.
Note: The * represents the tag non-breaking space tag that didn't show up because this board is html enambled. :)
I used to use a lot of non-breaking spaces. Then I read that it was best not to use them, because they'll "choke spiders." No idea if it's true, but I stopped using them, except for a couple here and there. I've seen sites' source code where it looks like multiple paragraphs full of of them.