[..]the underscore character is still indexed as a character
Hyphens are better than underscores, and you might as well use extensionless URL's...
www.mysite.com/heavy-metal-music would be the holy grail.
10:44 am on Sep 1, 2010 (gmt 0)
Without some modifications the holy grail is unavailable for wordpress sites because of the forced /category/.
If that's the case with your site try and keep a site structure that makes sense by leaving "category" (ie:/content/ in your example) right after the .com/.net/.org etc.
Again, it won't matter on a wordpress site because example.com/content/ is not a real category and returns a 404. I'd say wordpress needs a tad more seo love but it's widely used as is without too many problems so don't fret too much, just make sure not to change back and forth when you finally decide. :-)