Receptional_Andy - 6:20 pm on Apr 27, 2010 (gmt 0)
Although a plus symbol is a reserved character in a URL, it seems to be treated as a valid word separator nonetheless and IMO delivers equal relevance to a given keyword. The only SEO downside I can think of is that it suggests the pages are dynamically generated, which increases the chances of obscure pages/complicated URLs not being indexed.
I definitely wouldn't recommend changing a plus symbol to a hyphen for the sake of it on an existent site with established URLs unless there is some benefit other than increased keyword relevance in a URL you're aiming for. You'll likely end up with less total value even if you migrate URLs appropriately.