A couple sites have been testing the song sharing capabilities of the Zune. From their initial results it appears that the Zune will not share over 50% of the songs. This inability to share is not a problem with the Zune, rather the problem appears to be a DRM issue. It's pretty interesting to see how many songs on some labels will not be allowed to be shared.

UPDATE:  ZuneInsider.com has an update on this and a response from Microsoft on this issue.