Un-universal Plug n Pray
FWIW 'HTPC' cases are almost without exception FUGLY (cheap looking front-mounted tatty ports, anyone?) - or those that aren't are ludicrously expensive or are badly designed to the detriment of sane airflow. In the case of smaller Nvidia-powered SFF bricks they can (rightly so) be hidden behind the TV, mounted VESA style.
The concept of UPnP / DLNA is very nice & all but server software like that in MythTV needs to have a shedload of workarounds hard-coded into it to cope with many different clients. Apparently the 'universal' part of the acronym seems to mean "here's the standard, do whatever you want".
Besides, with a dumb streaming client you'll miss out on being able to schedule recordings but most importantly seek within streamed content (some clients can't even do that!) & do funky stuff like time-compress during playback. My wife *really* loves MythTV's ability to play her recordings back faster than real time & wouldn't be without her one button 4 minute skip feature.
Unless you really *must* have an optical drive in the livingroom you can still adhere to the 'no PC in the lounge' by using the existing backend machine as your frontend too. HDMI can be comfortably run for 10+ metres without an extender & extenders to put HDMI over 30 metres of cat6 cable are under £50 nowadays. Your remote control can be RF, or a local USB IR receiver (again, USB extenders don't cost the Earth) - or even just the one which comes with most TV tuners these days (obviously extended long enough to sit near your telly). I'll be doing this when (if?) we upgrade our TV - what's the point having 2 boxes when one can do the job just fine (I'd already have done it were it not for the fact you can't easily buy a PCI-E graphics card with s-video out).
Of course now with MythTV's APIs (http://www.mythtv.org/wiki/Services_API) on the cards it'll likely be possible to write a native mythtv frontend app for network connected TVs and possible to sideload apps the set manufacturers see fit not to approve for their walled garden. I'm pretty excited by the prospect - no more having to de-fuddle the virtually undocumented protocol, no messing about with MySQL... It'll be wonderful.