I posted a reply yesterday but screwed up and lost it before it got posted. I have SiliconDust HDHomeRun Prime matched with a Windows 7 desktop. I have a couple of XBox 360's in other rooms which let me have full control of watching live TV and recorded shows (as well as setting up new recordings), but WMC can also work well in a Apple-centric home...
There are apps like MC-TVConverter which will watch your 'Recorded TV' folder and auto-convert your shows to an AppleTV-friendly MP4 format. Obviously, there's a delay here...you can't watch live TV this way...the show finishes recording, MC-TVConverter starts converting it (which, for a 30 minute show, could be as fast as 15 minutes or so if you have a fast desktop), and then you can watch it on the AppleTV.
And/or, you can use something lie AirVideo's Server app running on the desktop box, and then use the AirVideo app on your iPhone/iPad to watch your shows. You still can't watch live TV, but you can watch something as soon as it has finished recording. And you can use AirPlay from the AirVideo iOS app to push it to the AppleTV, too. PQ won't be as good as the MC-TVConverter method, but it's decent. I wouldn't recommend pushing it to an AppleTV in a room with a large screen TV, and if you're watching it on the latest iPad, you won't be able to pull off smooth 1080p high-quality video, but it's suprisingly good. Well, I should add, that you need your desktop to have a capable CPU to do that on-the-fly transcoding at a high quality level.
Ideally, I'd love to see a solution where the shows get recorded as MP4 files from the get-go. I don't think Apple has much interest in offering DVR software, which is a shame, since they're pushing the iTunes TV show purchase model. Microsoft, who desperately needs their Windows phones to get traction, would be smart to have the next version of Windows Media Center allow for recording directly to a Windows Phone capable high-quality format, which would likely be MP4 under the covers, though also likely wrapped in a DRM container.