Looks like Western Digital is preparing a new version of their WDTV media player. The WDTV has similar innards as the SageTV HD Theater, minus the HTPC extender capabilities among other things. But it comes at a lower price (available under $99) and seems to have a pretty active user developer community. So now the WDTV 2 is reported to add network playback over ethernet, DTS audio decoding and a component video port to go along with HDMI. So it should be a little more comparable the the SageTV HD Theater if the reports are correct.
Speaking of the WDTV, you can get the existing version (not the unreleased WDTV2) of the WD TV HD Media Player at Newegg for $89.49 after applying coupon code HDDSALE15.