MS hasn't made it easy to enable more than two tuners under MCE. There are some tricky directions out there that explain how to enable four, which I tried, but it became apparent why MS doesn't make it easy, as my machine ran out of oomph trying to deal with moving 4 streams to disk.
Actually, I have the opposite impression, but this might be a recent change. Apparently they just updated MCE to address a lot of TV issues, and one of the things they did was to really seamlessly combine multiple tuners into one channel lineup. I'm almost positive that it's pretty easy to have more than two tuners now.
*edit*
Just check out
this post on EngadgetHD. The picture alone is encouraging.