The word "this" is not a preposition and should therefore be capitalized in a title. Since "Resource" and "Message" are capitalized, I know the buttons are going with title case.
I'm sure everyone knows what and where the "Discuss this Resource" button is. The "View this Message" button, however, is in the e-mail notifications.
I know this is trivial, but, again, I figured I'd report it.
Edit: "View this Conversation" as well. It's also in e-mail notifications -- specifically, the PC ones.