Hasn't stopped other distros from supporting Steam, with Valve's blessings no less. I use Steam on my Arch Linux machines with zero issues.

I also a happy user of Steam on Fedora. I can run full-screen games just fine (been playing Faster than Light recently).

It usually relies on the libc of the system too, which means you are REALLY limited to bleeding edge ubuntu for these normally. OpenNI has been horrible about this.

It really worries me, especially as it's not just differently structured distros like arch that are out in the cold, but debian, which Ubunutu is derived from.

It's bad. Ubuntu is the corporate partner for companies that don't want to do Free Software.

Currently using Steam on Linux Mint. Works as well as it ever has.

Mint being more or less a fork of Ubuntu, saying it works on Mint does not say much about the support for other distros.