My batch of Studio XPS systems are in, and I brought one home to play with.
I got the 2.93 GHz Core i7-940, 12 GiB RAM, 750 GB disk, Radeon HD 4670 512 MiB, Vista Ultimate x64, Blue Tooth, 19-1 media card reader, BD-ROM/DVD-DL writer, 802.11n wireless and an UltraSharp 2408 WFP 24" LCD monitor and 3 year next day onsite service.
...for less than the cost of the quad 2.66 GHz 3 GiB Mac Pro with AppleCare. (A similar quad Mac Pro is $4647, so the Apple tax is more than $2000 (MP 2.93 GHz, 8 GiB, 4870, 24" display, AppleCare 3yr).)
The system is very quiet, with it sitting on the table beside the monitor you can just tell that it's running (as long as the room is quiet - with any music, television or other activity going on you can't hear it). There's a power-on fan whoosh that others have reported, but that's gone as soon as the OS loads.
Pretty soon I'll have the T3500's as well.