Piece of cake... I actually have a multi zone control point single zone solution in my house like this (could be multi zone easily) using software all pre written for me...
APEJukebox is basically this solution and works with MP3 as well as APE lossless compressed files
http://www.apejukebox.com/ I dont use Ape Juke as it is not so good at distributed control...
The 4AM media consiole is another super sweet application that works great with a touchscreen computer... Again coded in flash and you really have to check out the GFX on this one...
http://www.overnighttech.com/products/4amMusic_overview.html again no distributed control (single control point) so I favour the following...
Media Jukebox has a plug in called web remote which allows you to control it from the PC its running on or other clients (as well as a neat solution called Netremote that does it from a PDA and can import Pronto CCF's to give you bi directional control over 802.11 using the pronto's GFX library)... I then run a flash client called Glissando that provides a nice attractive interface with track listings / access to the full album library.. Cover art etc...
I have built a RAID5 server to hold all my Ripped albums and many Ripped DVD's... I then use touchpad computers (over 802.11) or hacked Virgin Webplayers as control points... This all sounds expensive but the Webplayers cost $65 each and the touchpad computers I did a powerbuy on and sold about 100 at prices from 90 - 120 depending on how scuffed etc... Pretty damn cheap... The RAID server would not be needed for most people but I did not want to have to load up my HTPC with media files and after ripping and tagghing / cover art on ALL my CD's I liked the disk failure protection of RAID5.. Plus I have an always on server for X10 home monitoring / cameras and other interests...
I hope to be beta testing another distributed player in the next month that should be much prettier than glissando and more custom orientated.. I am considering if there is enough business for a whole house audio control solution based around some cheaper hardware i have found in my area...
Also if you want your wholeHTPC to look this nice check out MainLobby from Cinemar... You can make flash GUI's with dragabble buttons for program launches and control etc really easily