Actually Playnetwork is a sepearte company altogether, which provides services that are similar to those of Muzak. They are based out of Seattle. If I remember right PN was formed from the remnants of AEI Music that DMX didn't want when they bought out AEI in 2002.
(AEI were another Muzak-like service, actually founded in the early 70s as a response to Muzak, which were all-instrumental then. AEI conceived the whole "foreground music" concept--in fact their cmpany name was "AEI Foreground Music" until the mid-1990s when they dropped the word "Foreground" from their name. They were bought out by DMX in 2002.)
Playnetwork's delivery method, in general, differs from that of Muzak and DMX in that it is all on-site, meaning computers and CD-ROM discs. They don't have any satellite service of their own, rather opting to redistribute XM Satellite audio feeds.
@satech--
The PN computer definitely sounds interesting, and the specs you listed almost resemble that of DMX's higher-end Profusion systems (all of which are also dedicated Redhat-based PCs.) Does your machine use a micro-ATX form factor motherboard, like the DMX machines do?
Also, do the music files play back in your regular Winamp, or do you have to use some sort of non-standard (meaning DRM'd/quality-degraded) player programme?