Outlook is set default. If I ever click a mailto: link, it opens Outlook. I also have Windows Live Mail installed, used with my "funny mail" Live-address. For "real" emails I use Outlook.
I didn't have this issue with 2010 Tech Preview. It started when I installed this beta and now it is always there when Outlook is launched.
However, ir is harmless. I just click it away and use Outlook normally. It's just irritating, as I said earlier.