I asked the same question back in 2005.
http://forums.pelicanparts.com/porsche-911-technical-forum/257058-porsche-bubble.html
Boy was I early. This was after I sold my beautiful '67S SWT for three times what I paid for it. Oh, and I bought it for less than $10K
Kudos to the owner.
My broker has a saying "Sell them when they're yelling, and buy them when they're crying". Lots of yelling going on.
But it's this manic speculation that has driven me from the hobby. I haven't logged onto this place in months, and probably won't do so again. I've sold my last longhood, a one-owner '66, and my '74S.
I wait until there's some crying going on before I consider another 911. Until then back to the fun and value - 914's.
Bubble? Maybe once the bus is full, and everyone who wants one has one.....