GOG adds Mac support: http://www.gog.com/news/gogcom_goes_mac
Is Linux soon to come? Hope so...
Arch x86_64 User / Windows 7 x64 Dual Boot | Nvidia Geforce GT 430 | AMD Phenom II X4 925 (2.8 Ghz, Quad Core) | 6 GB DDR2 RAM | 500 GB HD
It will be a joyous day if they officially support linux, I have over $200 woth of stuff from GOG, and most of them work using PlayOnLinux software (which uses Wine), but sometimes it's such a freaking chore to set them up right.
Even if they cant (and most likely wont) recompile every game to run on linux, at least it would be cool if they could preconfigure a working wine setup (kind of like what they do with DOS titles using DosBox), so when you launched the program, wine would load up and take care of installation/execution without having to manually set it up yourself(hooray for PlayOnLinux).