Thought I'd post my recent advances here too, as it's the most relevant thread!
I had problems playing Hummy recordings from the Pi, until I found out that I hadn't unencrypted the recordings! nothing more to install (Don't need to install the app 'unencrypt', dont' need to install MediaTomb) Just browse your recordings and click Opt on the folders, and enable Unencrypt on each one. Don't forget to also go up one from your main directory so you can enable it on that to. Not sure the time frame for tr hummy to busy itself doing the conversion, but once a file has a bright green 'Dec' next to it, it's been decrypted and plays fine over the network.
I also attached my USB drive full of videos to the Hummy (Originally it was going to be attached to one of the Pis and shared). The Hummy runs SMB (aka 'Samba') which I tried installing on the Pi but it kept crashing. On the hummy it's perfectly stable, so the Hummy now acts as the media server for all my media, and on the Pi you 'Add Videos'>'SMB (Windows)' Network> And find your files there through the Hummy.
The Pi has a really nice interface in Raspmc the only gripe I have is fast-forward and rewind are useless, so instead I've remapped the remote so instead of fast-scanning through the video, the FF buttons skip 20 seconds. I find it works pretty good now.
I won't go into great lengths, but if anyone would like some Pi advice I'll try and help, but take a look at
http://forum.stmlabs.com/forumdisplay.php?fid=7
My tweaks have been to remote.xml, (to remap remote buttons to do what I want) advancedsettings.xml (To change how far FF skips) and little overclock tweaks to config.txt (Though I havent noticed any difference! - just saw them online and thought it wouldn't hurt to add a few lines of code!) These changes can be made using SSH connection, (Using 'PuTTy') I'll try and advise on the syntax (with my limited knowledge) if anyone needs!