File this under: not totally clear
The new Zune Desktop v4.7 client came out on Monday, adding of course Windows Phone 7 support and a few other additions related to video rentals/purchases. While most have been happy with the upgrade, it looks like a few users are experiencing potential memory leaks related to the media center app--but it may be isolated to just 64-bit editions of Windows 7.
ZNet is reporting that Microsoft is aware of the issue and investigating, while users in their forum look to find a solution. If you're experiencing such problems, there is a media-collection reset tool that may help with the problem.
On our end, running Windows 7 32-bit, the memory seems stable to use, with Zune v4.7 using a reasonable 58.8MB of memory and holding steady (by comparison, our Firefox browser is sucking 245MB, Chrome even higher)