I had to do some analysis on this recently. Yes, Boxster sales are down. But from what I can tell, a main reason for the decrease in sales is because the hard-top Boxster (I think it's called the Cayman

) has cannibalized some of the Boxster's sales. If you're in the market for a new Box, now is the time to buy.
But I don't see this as a death knell for the existing product line. I do expect Porsche will drop manufacturing volume in 08 and beyond, but other than that, I highly doubt they'd consider axeing the line at this point.
But if they did, it would likely have a positive effect on the value of our cars!!