Mark,
I've noticed these auctions, too. XO-1s, RB-Ts, RB-2s, and MB-1s are pretty hot items, too.
I ride an RB-2 that I bought new in September 1992, and while it's a great bike, it's not, as you say, a hand-made frame. (I've never had a hand-made frame, but I it's not hard to figure out why a hand-made, personally-fitted frame, is obviously superior to a production frame.)
I don't know if it's still up, but Grant Peterson addressed this very thing
in a FAQ at the Rivendell site. (Yes, I just checked, and it's still up:
http://www.rivendellbicycles.com/
But I suspect that the popularity of the best of the Bridgestones is analogous to, say, the popularity of the Peugeot PX-10. The PX-10 wasn't a hand-fitted frame, but it's got a reputation of a very good bike for the money. And, like the Bridgestone RB-1 and RB-2, it's not made anymore, and eBay bidding will reflect that. I think a lot of other models sought-after on eBay probably fit into this category.
The Bridgestones, though, are probably thought of as more connected to Rivendells than they are, maybe as a poor-man's Rivendell. Even so, I think the prices on eBay are driven more by the aura of the Grant Peterson Bridgestone era than by the reality of the Rivendell marque. Just like anything else that's no longer made, the upper-end Bridgestones have their collectible value.
But that's just my take on an interesting phenomenon. . . .
nath "one day I'll have a Silk Hope" dresser