I think it's a combination of both....
Yes the Boxster is losing some cache due to it's age and is getting serious challenges from down market. It did not have a new model intro in Fall whereas the 911 did. That's part 1. Part 2 of the drop could be defined by it being the least expensive car in the line up. As such, it's the car that can pull tertiary customers up from less expensive cars. ie. the typical mid life crisis buyer looking at a Miata, S2000 or 350Z b/c he/she can 'stretch' and afford one by cashing out some equity or taking a long loan, etc. However, since the mkts have tanked, their homes have lost value (not many haven't) and they either stick down the price stream, or forgo the purchase entirely. So, it's a combination of both - part 2 is bad enough on it's own but it exacerbates the problems created by part 1. Remember, Boxster sales were weakening before our current 'meltdown' (also explains all the SE, LE, and design series that were released to try and stave the drop off as much as possible).
The 911, as stated above, benefited from a revamp in fall to the 997.2 (w/ IMS-less motor) and the release of the 997 Turbo Cab (yes in 9/07 but not really commonly avail until '08). Sales will be helped up as much as possible by GT2 and GT3s. The 911's sales drops will be more a pure function of the financial situation and car buying in total. Even the pepper is starting to show signs of that (whereas they bucked the trend even when gas was 2xs as expensive).
The plus (for buyers) is how much the used mkt is coming down on all of these - of course that's a big big minus for us current owners but no one is immune. Personally, I think you'll see 911s will start taking some big hits in depreciation (as Boxsters already are/have). I'm seeing way too many 997s in the used mkts. In fact one local dealer has 12 (four - '08, five - '07, two - '06, 1 - '05). Under 'normal' circumstances, first off they would have only 1/2 that many, but more importantly the MY spread would be reversed.
JM2C - YMMV
