As long as it stays in non-free reps...

As long as Mono stays where it belongs (in the "non-free" repositories), it won't be a problem. Sure, MS can (and probably will) get their lawyer on the back of any successfull open-source project writen in C#, but if it doesn't include anything critical it should be OK. Just pull the plug on the affected project. That's why OSS people should refrain from starting time-consuming projects in C# (it will be time wasted in the end, I think it was Stallman's point), but getting the Mono thing in the non-free section to provide support for the odd small gadgets won't hurt. Probably. That's what the non-free repositories are for, after all.

And for the ones who spouted nonsense about Ubuntu, I think you'll find that this particular Debian's bastard child is considerably less free than its grown-up parent. It'll presumably be the first to go when shit hits the fan. And starting from Debian and it's rock-solid, almost universal hardware support, it's second-to none package management system and it's fuckastically huge package repositories was probably the brightest idea Canonical ever had. I mean, when you're looking for a free, just-work-out-of-the-box base with a software package or ten for every possible need, what other choice do you have nowadays? (the first to shout "windows" will get a sharp kick to the gonads. Same for "Apple OS"). Maybe Gentoo*? But seriously, who in the luser world is gonna compile their own system from the ground up?

*Quite sadly, I'm being serious here.