It seems like there's been a gradual appreciation of upper-end bikes, but I wouldn't exactly call it a bubble except, perhaps, in the case of post-Ernesto Colnagos. You could write it off to the bling, I guess.
On the other hand I can't understand some of the prices being paid for thoroughly mediocre low- and mid-range bikes from the mid-70s forward. And this cult around bike-boom Raleigh Super Courses is totally over the top.