Fusion 4.1.3 would work on my MBP so I downloaded the 2012 Technology Preview. That seems to work fine with the DP's and GM release. Shame I've lost the ability to virtualize 10.5/10.6 & 10.7 as you could in Fusion 4.1.0. I should imagine VMware will release a compatible version soon
I do not see the problem here. If the applications are not signed and do not run under gatekeeper, then turn it off. You didn't have it in Lion and you were fine, so if it inhibits your ability to use your computer, like blocking applications you need, then you can turn it off in Mountain Lion.