UPDATE: Obama just returned to greet HRC, who was speaking to Sen. Patty Murray. They were talking and laughing. Obama was completely focused on Clinton, who put her bag down (as if to say 'I'm going to be here for a while'). Clinton then busted out in her classic chuckle-cackle and Obama chuckle-cackled right along.
They were quickly joined by other senators -- Feinstein, Boxer, Durbin -- who joined in on the unity-building fun.