The actual density for your page will depend on how the search engine in question treats stop words. Google has a list on their site some place, not sure about the other SE's.
Depending on the stop word list, you can come up with some very different numbers.
It's a good general guide when you run your site through one of those things, but if you do feel the keyword density is too high, perhaps try rewriting some of the content?
23% does seem a bit much. Are some of those instances in links? Is the page large or small, relatively speaking?
If the instances are in links, then I've seen higher than that still do well in Google, at least.
For Inktomi, as far as I've been able to tell, a smaller density would be better.