I spotted a thread on the Mac OS X Forums that discussed this very topic. Forum user "Iambob" took it upon himself to write a GUI wrapper for nice, which would present all the jobs with their current priorities, and allow easy tweaking of the values. Quite logically, he named his program nicer.
You can find nicer here, and I must say it's a very useful progam. There's some work he'd like to do relative to optimization, but the basic functionality is there, and it's basically self-explanatory: double-click a process to change its priority. It takes a while to launch, but don't worry, it's not crashing your OS.
I think this program is a great example of what we'll see as we progress towards OS X 1.0: GUI wrappers around the core UNIX commands to make them more useful to the typical Mac user. Thanks, IamBob!
[Editor's note: Version 1.0 of nicer is now available at the above link!]

