one of the best episodes, though the notion of a "constant" and especially the way Daniel expounded it doesn't really scream "sci-fi" (then again, was the show ever supposed to be "pure sci-fi" ?)
btw. Desmond is Daniel's constant ? I thought Daniel said the constant had to be something both 'familiar' and which the subject 'really really cared about', manifestly Desmond is neither of these, so it turns out the constant merely has to be a physical reference common to both timelines
also, did Desmond recover his memories or is he back in 2004 with an 8-year amnesia ?