The current plan is to get the Windows version pretty stable before having to worry about the problems that other OS's will bring up. Since Unity is a completely cross platform engine, once e build has been selected as a stable candidate for windows, the Mac and Linux versions can simply be compiled (I am unsure if they will be released together or one at a time), this will undoubtedly bring to light many more bugs that only exist in the Mac and Linux builds, so I hope you understand the reasons for going Windows only for now, simply to prevent being flooded with many more bugs, lots of which may only exist in one build.
I assure you that the beta is planned to be released on Mac and Linux eventually, and any help testing by Mac and Linux users will be greatly appreciated, as there are fewer of you, each of you is more valuable as a tester.
If you can run a Windows virtual machine on your Mac or Linux machines, you can join in on the Windows Beta right now.
That's unfortunate, while I still have a dual boot setup I usually manage to boot windows only every few month (which then reminds me why I use linux). Is there a way to sign up for news about the state of linux/mac versions? I would also be happy about an 'as-is'-version outside of steam as it was possible for the little demo packages before.