Here is a piece looking at what's known about the next release of OS X, 10.6 aka Snow Leopard:
Mac OS X Snow Leopard 10.6 - the story so far | News | TechRadar UK
The focus is largely on performance -- graphics, multi-core, etc. Which raises one question with me. What's up with OS X grinding to a halt whenever there's any I/O?
By which I mean... when copying files from one disk to another, or pictures off a compact flash card, or Time Machine starts doing a backup... everything seems to come to a stop. I might expect that some other I/O-bound process could become twice as slow, or even 5x as slow if there was contention on the same disk for writing things like browser cache files, causing seeking back and forth.
But my perception is that it's worse than that. Browser windows freeze completely until Time Machine is finished. Operations slow to a crawl when I'm pretty sure they're not touching the disk that's being copied to or from. Pictures take forever to come off a compact flash card when nothing else is tying up the processor. You would think it would be an obvious optimization to make Time Machine run at a low priority when the foreground app was doing I/O, or to speed up copies between compact flash cards and Firewire external drives, where the internal drive shouldn't be involved at all. Yet in this one area, I find Windows beating OS X in speed.
I don't know if it's something deep within BSD, like you periodically hear some bugaboo in Linux internals that makes certain operations really slow. I've seen these slowdowns when copying or moving files both from the Finder and the Terminal.