There are probably quite a few OS's that would run, (not windows 8 though – IMO).
The problem is getting someone to port the OS to the device. It generally not an easy task, and it won't be done by the Foundation (although help will be offered if necessary and it helps the foundation)
I'm also interested in using Raspberry Pi to use for Web TV stuff. Really the only obstacle I forsee in doing everything you want is the ARM CPU. I know ARM is very popular for mobile devices, but that's pretty much the only place it's popular in. So unless you install a mobile OS like Android or Windows Phone, you're going to have a hard time finding compatible applications and drivers to use unless you program your own which is kind of the purpose of the Raspberry Pi. All you really need though is an OS that supports Adobe Flash and modern web standards. That'll let you browse the web and watch a lot of video content.
I think Arm is going to get better market penetration over the next couple of years.
Compatible programs are MUCH more common that you state – but they are all Linux ones. The Debian archive contains thousands (?) of apps recompiled for Arm that work fine on the Raspi.
Also, I have succesfully compiled x86 Linux apps to Raspi Arm with no problems. Slow, but got there in the end.
Well, I guess the only applications I really need are drivers for peripherals like keyboard, mouse, wifi dongle, etc and Adobe Flash for watching web content. Unfortunately, the reality is that HTML5 hasn't really penetrated web multimedia yet and Flash/Silverlight still dominate the web. Correct me if I'm wrong, but I do believe that there's an ARM version of Adobe Flash for Ubuntu. Not sure if anyone has tried it out yet though.
As for Windows 8, I'm assuming it's pretty similar to Windows 7 under the hood, if not more streamlined. I read an article a few years ago that someone managed to install Windows 7 on a Pentium II PC with a minimum of 96 MB of RAM. But 128 MB was better. He also got it to run on a Pentium I CPU with similar RAM and 2 MB Video RAM. Here's a link to the article:
Clearly without a doubt, modern ARM CPU's including the ARM11 ones used in Pi are a lot more powerful than Pentium I & II CPU's and should have more video RAM than 2 MB. So if Windows 8 is anything like Windows 7 under the hood, then it might be able to run relatively well on the 256 MB version of Raspberry Pi. Now I don't know about performance though, I read other articles online that people tried a newer Pentium III PC and apparently it took 17 hours to install and almost 20 minutes to boot. But other people claim faster times of 1 hour install and a few minutes to boot. Windows 8 is designed to have faster install and boot times on weaker hardware.