bash is simply an executable program located in /bin. You can download the source code for bash, then compile it yourself and set the Terminal and/or X11 to use your new executable, and it's completely possible to use bash 3.0 (or 3.1) as your default shell in Mac OS X without "massing the whole OS up" or causing any harm to your system.
I don't see why one would think that this would somehow "screw up" or "mess up" Mac OS X in any way... it's simply a program, which you can run without incident just like any other program under Mac OS X. It's not buried deep within some cryptic place in Mac OS X, it's not intertwined with the OS in some way that could cause harm to your system, and it's not going to prevent you from using the Terminal and relegate you to using X11... it's
just a program, and is a pretty ubiquitous program with Unix distributions.
You can download the source code from
http://www.gnu.org/software/bash/ and compile it yourself and give it a try. It is completely possible to do. If you want and need bash 3.0, it is even adviseable. You can even keep your old bash 2 in the same place (/bin), and install bash 3.0 in a location such as /usr/local/bin or something like that and flip-flop back and forth between the two if you so desire.
Edit: I just downloaded bash 3.1 from that site, configured and installed it in less than 15 minutes on a 500MHz G4 machine, and my Mac OS X installation is as perfect as it was before I did it -- nothing catastrophic happened... in fact, nothing at all happened except for now I have a bash 3.1 program, just as expected. It was simple, required me to type three things on the command line, and the rest was pretty automatic -- all in all, I say go for it. If you need help getting this done, I'd be glad to post a step-by-step on how to do it.
