Not true.
Porsche have sold extremely small numbers of new cars over the last 2 years (due the credit crunch). Consequently, used supply is extremely constrained. Therefore, used values have gone up considerably recently.
In the 6 months I was looking for a Porsche 911 (until I opted for the R), used prices went up over 10%.
I'm not convinced it's due to lesser numbers - my local dealer has not noticed a slowing of new Porsche sales.
The announcement of 'big' road tax prices seemed to have confused the stupid into flogging off their £70000 cars cheap..instead of paying £400+ per year in road tax. Madness. Regardless, 996 and 997 prices fell through the floor overnight.
As you say though, they recovered...big time.
I recently sold my 2004 C4S and got £7000-8000 more than I'd have been offered 12 months earlier.
996s and 997s are still a bargain though, if people can accept the running costs....you may be able to find faster in a straight line, for less money...but I doubt it's possible to find a more involving car for the coin.
I loved mine...and convinced myself that I wanted something silly for the weekends, so sold my S3 and went C4S.
After 5 years, i've now decided I want something 'nice' to use everyday thats practical and fun...full circle.