SIt only takes hearing so many jokes about Al Gore inventing Twitter to figure out that the former vice president has signed up for the microblogging service. Wisely, he's not really participating in the site, just using it to market his websites and announce his interview with Digg founder Kevin Rose, which airs tonight on Current, the Gore-backed cable channel. Current and Digg have been teaming up for a series of election-related events, including a party on election night. But Rose and Gore's acquaintance goes back almost two years.In late 2006, Gore's Current made an offer for Digg which valued the social-news startup at $100 milion or more. Wonder if Rose and Gore discussed business at all in this interview. As VentureBeat recently pointed out, Digg's traffic is flat, and it hasn't significantly increased its valuation since Rose and Gore's 2006 chat.