I'm not exactly sure why, but in Snow Leopard, clipping a mail address also copies the addressee name. So when you paste it, you don't have a mailable address. Why Apple changed this? I have no clue. Thanks to Hawk Wings, I found the solution today.

Open your terminal and paste in the following:

defaults write com.apple.mail AddressesIncludeNameOnPasteboard -bool NO

Problem solved.