There is most definitely a performance-hit involved with this (I'm not lucky enough to have one of the forthcoming hybrid disks).  I use a Lenovo T60p to do software development for the Jboss application server.  The specs (that matter) are:

Core Duo T2500 @ 2.0 GHz
2 GB RAM (DDR2-667)
100 GB 7200 RPM SATA HDD

Under plain Windows XP, the time to bring the server up and deploy the webapps is about 45 seconds on average.  Using Windows Vista (the drivers installed between the two OS's are as comparable as I could get them to me), the exact same process takes 65 seconds on average.  That's almost a 50% increase, and the only real variable between the two is that Win XP is a non-transactional filesystem, while Vista is transactional.   

Starting the server and deploying webapps is a highly filesystem-bound operation, as my desktop with the following specs:

Athlon 64 X2 3800+ @ 2.4 GHz
1 GB RAM (DDR-522)
2x 37 GB 10,000 RPM SATA HDD's in RAID-0

...can complete the same operation in 35 seconds on average under Windows XP.  

Benefits be damned, I'd prefer the option of reverting Vista to a fast, non-transactional filesystem.