Re: CAD software
We are pretty much a Linux shop, with a few Solaris machines being retired as fast as we can get our gonads out of Larry's money-making vice. However we do require Office for some document work where it has to be format-perfect with other MS users, and a few CAD packages that are Windows-only. Our solution is to use Linux for our desktops and have a few VMs with Windows for those applications (often XP, as for 7 you need the enterprise license I think for virtulisation), and not give them internet access.
Result is pretty much the best of both worlds in terms of cost, flexibility and security. Yes it adds a slight complexity to using software in having to fire up the VMs but for anyone capable of using CAD software or writing complex technical or business documents its not a challenge after 10 mins of tuition.
Of course it might not suit everyone's use-case, but is worth considering.