A theory I came up with a few years ago, was that the constant use of nuclear energy caused Sam's true body to dissipate, leaving only the kind of "ghost" that Stawpah (most likely misspelled that) was in MIrror Image; that Sam's actions hadn't gone unnoticed, thus as a final reward, he was given the ultimatum, to help himself and return home, or help Al by preventing Beth from remarrying.
Think about Stawpah, and the way he seemed to know about the trapped miners, as though he himself had once leaped into one, or both, of the miners.
Being a former miner himself, this might've been HIS last wish, his final request to the bartender Al.
Still, the closest thing we have to the absolute truth; Bellisario's original ending, before Warren Littlefield (the turd who ran NBC at the time) cancelled the show; that being, that Sam's leaps took him into the future, and from that point on, my guess is either his intent was to change the "Sam's Lifetime" rule, allow Sam only to leap up until the year of his death, or use the exception to the lifetime rule; just as Sam leaped before his birth, into his great grandfather, he would leap into the future via his daughter, and her offspring.
I've often wondered, about the sad ending-"why?", and the only answer is that Bellisario had split up with Deborah Pratt, and, left in that "no happy endings" mindframe, decided to turn Sam into a martyr of sorts.
Okay, one final observation about the final episode- the more I watch it, and see the return appearance of actors throughout the series, I begin to wonder that maybe Al's Place is an illusion created by Sam's mind (though this offers no explanation as to how Al can find him by searching the year of his birth) since it IS Sam who, allegedly, has subconscious control over his leaps (perhaps why he was able to leap into several people for one task, in "Double Identity" and in the episodes I sometimes refer to as the Abigail Trilogy)
It's the only reasonable explanation, that I can think of anyway, as to why Sam would be holding a wallet with an ID from the future.
And it'd explain the recurring actors, with appearances familiar to Sam, and identities, for the most part, foreign to Sam; that he was having, understandably, an identity crisis at this point, where his "swiss-cheesed" mind was trying to sync up faces to personalities- why?
Because, by that point and time, Sam's subconscious realized what he had to do for Al, and for Sam to "evolve" as it were, to no longer needing to use the bodies of others, he first needed to remember what Beth, and her location, looked like, and thus his mind went through the faces and personalities he'd encountered in his travels; some matched their true identity (Jimmy, known as "Pete" in Mirror Image, and his brother) while others were way off (Captain Galaxy, named Ziggy)
When Sam hears that, he begins to work out in his head who Ziggy and Captain Galaxy really are, thus filling in the holes of his swiss-cheesed brain.
Plus, there's the fact that in spite of being Sam's birthdate, the people whom he recognized appeared the exact same as they had when he originally encountered them.
Still, the flaw in that theory is that Al (Calavicci not the bartender) can still find Sam.
Wow, thinking about the thousands of possible interpretations for that episode is making my brain hurt- and REALLY making me wanna watch that episode again(gotta find the copy I recorded onto a DVD-R off of Sci-Fi- screw the DVD set, when they removed "Georgia On My Mind" from M.I.A. I lost all faith in TV DVDs)
Still, if he leaped back into himself, where'd the fermi suit go? (That tight little white number that drove women, and bi guys like myself, crazy