I'm stunned that Dempsey made this move. Obviously there is more to the story. He's in his prime, captain of the national team, and was hungry for CL football before his move to Spurs. I bet Klinsmann is not happy about this but it's not his decision to make.
As for other Americans that could leave MLS for Europe, Juan Agudelo and Omar Gonzalez's contracts run out in January they will both look to move to Europe. Under the radar would be a guy like Luis Gil. He's 19 and was probably the best US player at the U-20 WC last month. I think his contract also ends either in January or next summer.
Part of the problem of US youth development is that guys aren't ready to make the jump from MLS until they are in the 24-27 range. Ideally the US has a structure where they sign at 18, reserve team at 19, make the bench/spot starts at 20, starter at 21, and dominate/move to Europe at 22. College soccer has limits on practice time while their rest of world counterpart is practicing in a pro environment everyday.