The BlitzMax info page said it was available for Mac and the purchase button lead to a page on ShareIt saying you are buying the Mac version.
It also stated back then that you would have access to the PC & Linux versions as they get released at no extra charge, and I'd assume Syb was paying for that ahead of time, with the beta's being a little bonus to tinker with.
As for the BMax GUI, I'm assuming that would mean a B+ update would be around the corner no? Skid' has mentioned a couple times that both programs are working off the same GUI base or something.