Basically everytime it hits 100% mplayer loses a/v sync. With a docu where you don't see the speaker it is not that obvious, but when you watch a full movie then after a certain time it is more or less unwatchabe. This is mainly with pure m2ts as the bitrates are really high. The same problem is even with vdpau when the stream has a few errors then it loses sync. Common win players do not have got this problem. I think mplayer should be optimized to resync better.
I've resorted to a multithreaded mplayer for my media center until GPU acceleration happens. You just have to check out the current mplayer and current ffmpeg-mt, then replace the libavcodec, libavformat and libavutil in the mplayer directory with the ones from ffmpeg. Configure, compile and install. When you run mplayer, just do mplayer -vfm ffmpeg -lavdopts threads=2 (or however many cores you have) and you're off and running. Your Core Duo should be more than fast enough... my 2.17GHz Core 2 Duo has a fair bit of overhead left when running 1080i mts files.
Another option to ffmpeg-mt is to use coreavc. Yes I know, proprietary, closed source, non-free, windoze binaries... but it actually *does work*, and for that matter, VERY WELL. coreavc is multi-threaded, and is (in my experience at least) quite a bit lighter on the CPU than ffmpeg-mt. I also notice that audio *always* stays in sync, even if it tops out the CPU.
Ubuntu forum, but the instructions are universally applicable -- I did it with F12-Alpha. For that matter, I took the rpmfuzion mplayer.src.rpm, added in the coreavc stuff, and made a binary rpm out of it so that it doesn't pollute my system.