I've always thought HTML in the title tag was a bit amateurish, and I know I've seen it displayed raw in some browsers. However, I just noticed that almost all the top dogs for the 5 search terms I checked are using <b>keyword</b> in the title. I guess that's OK now?
I'd be careful with that. I remember when google used to display arrows in the title (It was a specific unicode character). I tried it out on a small site and it didn't take long before Google caught on to the trick and the site dropped in the rankings for it.