Oooh, I don't think Apple would like that one bit. Without complete control over what code gets pushed out through their update servers, people would begin pointing the finger at Apple when Software Update borked their FireFox installation or something because the folks at Mozilla made a mistake or something...
In addition, that would put the burden of security-checking ALL code on the Software Update servers on Apple, to ensure that some rogue, disgruntled employee at some 3rd-party software company didn't get all pissed off and push a trojan out through Apple's Software Update servers.
Just my $0.02, but I don't like the idea one bit. It sounds super magnificent and awesome, but is a GREAT security concern for both Apple and me.