I was actually about to start a new thread on this very subject, but since this one is already here I guess I'll piggyback...
This is what I'm considering:
Mac Mini server, 2.0ghz i7 quad-core
+ 16 gig RAM upgrade
+ 256 gig SSD upgrade
I'd add the RAM/SSD myself, not via Apple. Probably the Samsung 830, sounds like it's well behaved under OSX and good with incompressible data. And I've got a local independent Apple shop that can swap drives for me if I decide it's too fiddly for me to deal with myself.
The one thing I'm hung up on is, the "server" Mac Mini doesn't have a dedicated graphics card, just the Intel on-board graphics (HD 3000). Would it be wiser to get a dual-core i7 at a higher clock speed and with a dedicated video card? I'd still do the same RAM/SSD upgrades. Does the graphics card really matter? LR4 performance is the most important thing, and running Photoshop well would be a plus. Compatibility with future versions of LR is also a concern.
Edit: I'm not using a D800 or anything crazy like that either, most images would be in the 10-16mpx range.