See my other comment with link to format/container support.
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Play To works with all media - music, pictures, videos, and recorded tv. There is a table of the containers and formats natively supported by Windows 7 on the Engineering Windows 7 blog: http://blogs.msdn.com/blogfiles/e7/WindowsLiveWriter/MediaStreamingwithWindows7_12FCB/image_54.png
Any media codec/container will be supported in a transcode operation so long as it plugs into the Media Foundation (MF) platform. The rule of thumb is: if it will playback in Windows Media Player, we should be able to transcode it into another format playable by devices. As you can see from the above table, Windows 7 already supports that vast majority of containers and codecs, and can transcode between them.
Note; however, that Windows 7 only transcodes into lower resolutions in various formats; we don't upscale. This becomes particularly important for HD content. We always offer the native format/resolution to a device; however, if a device does not support the native format/resolution, they will only be offered SD versions of the file in various other formats and resolutions. A recent example I came accross was an AVC-HD file. The TV I was streaming to only supported MPEG-2 PS, so Even the file was available in all its HD glory, the device could only play DVD quality MPEG-2 version of the file. I loved that the file played back because we could transcode it; however, I wish it was in HD.
When you're making purchasing decisions, look for products that have the "Compatible with Windows 7 logo", which means it is both DLNA certified and meets a bunch of Windows requirements, and also supports a bunch of formats natively.