In leaked documents, Microsoft has outline the development platform for the Windows Phone OS and the key development environment for Win Phone games will be XNA.
XNA is the development environment for both XBox LIVE and the ZuneHD. This appears to confirm that the XBox LIVE games will be ported to the Win Phone platform and suggests that ZuneHD, XBox LIVE and Windows Phone will all be compatible. XNA support guarantees that there will be a wide selection of high quality games available at launch of WinPhone.