My 2 cents: Steinberg made Wavelab Mac OSX capable, because they saw the added business. There are some DAWs that only work with Mac, and almost none that work with Linux. Windows is still, to this day #1 in the OS market. Apple is (or was) the world's largest company, forget just a computer manufacturer. These are just the knowns.
I believe it would make sense to add staff to port Sonar over to the Mac. If you're ever going to get a Pro-Tools user to try Sonar out, this is a good way to do that. Unfortunately, Apple's direction is very secretive, which makes their products unpredictable to some extent. Microsoft rarely is secretive to that extent.
Cakewalk can decide to make Sonar for the Mac, and I suspect this could be something for X3. They would increase their sales, I believe, but the hardware IS expensive. Usually, anyone who buys a Mac Pro, for example, has the $ to go the ProTools route, not taking chances on something new. If it were me, I'd lean heavily toward PT for the Mac. I live in Nashville, and all of my friends who have Mac DAW's use PT or Logic. However, they would give Sonar a try on the Mac, sure. They just wouldn't reach for it as the first one to grab when purchasing a $3,000 Mac Pro. If Cakewalk wants Sonar on the Mac, they'd better get started soon, and they will definitely need to work out the kinks. I find Apple users far less tolerant to bugs than Windows users.
Ubuntu can forget it for now. Unfortunately, there is a very VERY tiny DAW market there, and most studio engineers would rather not have to deal with Linux. They don't have time. Ubuntu is rock solid, and I love it (I use 10.04 and 12.04 LTS servers every day), but it's still got a ways to go to be really DAW and DAW hardware-friendly. Believe me, I wish this were not the case.