Pluto Home is the project that LinuxMCE is based on. They are essentially the same thing, the different is Pluto Home is supposed to be a black box application that takes over your computer (with the advantage of stability) whereas LinuxMCE is designed to coexist with everything else a little better. Pluto Home is also offered as a commercial solution preinstalled and configured (but the software itself is still open source).
To get more info on how it all worked I visited the Pluto Home forums.
It seems that MythTV isn’t as integrated into Pluto Home/LinuxMCE as I’d hoped, but it might not actually matter depending on how you use it. When you select TV, it just launches MythTV and puts it at the front. It then simulates key presses from whatever your current control method is (which could be a remote, a mouse or their Symbian software on a bluetooth mobile phone). If you can use MythWeb to set things to record (which is what I use with Myth most of the time) and then just use Pluto/LinuxMCE to view the recordings then minimal integration is not a problem since I’d never use MythFrontend…
Pluto Home, Linux, MythTV, LinuxMCE, MCE, media centre HTPC