i'm not really used to OS X programming but, can't it be written in a way that the compilation process differs from the versions in the same code? i mean, put all the code together, then with compilation flags turn the 10.6 features on or off depending on the target system.
i guess that turning mplayer into 64bit and taking advantage of Grand Central Dispatch will be quite a rewrite in the code, and since mplayer devs are a bit lazy on the mac side IMHO (i say this because it doesn't seem to have so much support as the other systems) i don't see that coming unless some team starts rewriting mplayer from ground zero
