For now. In theory, with enough emulated libraries, you could probably run anything with this. But I don't think the point is to run "random mac app" on Linux: it's probably more likely to enable something like iPhone development, which right now is Mac only.
I don't consider the 3.3.1 situation to be relevant, as in this case you would be running the official Apple toolchain.
If you had brought up the section of the iOS Developer License Agreement relating to "non-Apple-branded hardware" you might be in the right ballpark, but the legality of a similar clause in the Mac OS EULA was cast into doubt in the Psystar case. Psystar aside, the (non-commercial) OSx86 scene continues to thrive, unhindered by Apple.
At the end of the day, given that you would be running the official Apple toolchain, how would Apple be able to prove that a particular iOS app was not built on a Mac?
However, GCC and friends are licensed under the GPL: the code is published at http://opensource.apple.com; their only technical barrier so far is actually making it run on another OS.
That's not to say they couldn't block something like that, but with GPL code it does become significantly more difficult, if not (technically, not "this would take forever to port back to Linux") impossible.
I agree that this is a fair piece of precedent, but I would also like to note Apple eventually removed this clause and developers are again free to develop in other languages.
I don't know how exactly it all comes together, but I would hope that Maloader and Cocotron (http://www.cocotron.org/) would enable many Cocoa apps to run on Linux (on Intel) without recompilation.
From the Cocotron website: "The Cocotron is an open source project which aims to implement a cross-platform Objective-C API similar to that described by Apple Inc.'s Cocoa documentation. This includes the AppKit, Foundation, Objective-C runtime and support APIs such as CoreGraphics and CoreFoundation."
