The PVR branch of XBMC with the addon for MythTV (libcmyth) works quite well for me ( http://forum.xbmc.org/showthread.php?t=110694
) but "timeshifting" live tv hasn't been implemented yet. Alternatively, you can use the Mythbox addon which has a different-looking UI so IMO doesn't look as good as the PVR addon which integrates into the theme. It has some extra functionality, though, like pulling in TV show banners in the EPG which is nice.
Another backend alternative to MythTV or TVHeadend is vdr which also has a couple of XBMC PVR addons. I haven't fully explored it yet, but it will require some extra steps getting the EPG set up. You'll have to set up XMLTV with Schedules Direct then use a vdr plugin to convert the data and so on from what I've gathered so far.
As for TVHeadend, I never was able to get a good channel scan from it, and it won't use a channels.conf file, but I was scanning QAM cable channels which is a completely different beast than OTA, so it might work better for you.
RE: Adobe Flash Player 11.2 beta with multi-threaded video decoding support -- I finally got around to trying it, and while the performance is *better* it still isn't *perfect*. It will choke on me now and again (this is with an on-board ATI HD4200 with current proprietary drivers, btw). I really don't watch enough Flash video to care.
I knew as soon as Hulu started testing HD streams that it would only be a matter of time before they went to a subscription model and seriously limited availability of their "free" content. Funny enough, you still get to watch all of the commercials with a paid subscription to Hulu Plus.
Anyway, it's pretty straightforward building XBMC with PVR and MythTV support from GIT if that's the route that you choose. I might even write a little script to do it. I'll post it if I do.