As an update to this thread, I went ahead and bought a Seagate Momentus XT hybrid drive for my Dell mini. I upgraded the drive before installing OS X, so I can't give you any comparative anecdotal observations, but I can say that I am VERY impressed with the performance of my machine.
I'm actually blown away by how well this works. I wanted to put this machine together as a travel machine for importing my photos using Adobe Lightroom. My intention was to import and catalog while traveling, then import the catalog into my iMac once I got back home for final processing.
I was pleasantly surprised at how well this works. Import and flagging the good photos works great, and even using the develop module is more than acceptable performance-wise. If it weren't for the limited screen space, I would otherwise be very comfortable using this machine for moderate use all the way through from camera to final JPG. Posting the odd photo from the mini to my blog while on the road is certainly doable.
That said, I don't know how much of this is thanks to the hard drive upgrade, the memory upgrade (I also put in 2 GB) or just OS X itself. In any case, if anyone is considering the hybrid drive for their setup, I'd definitely recommend it.