I'm planning to buy a Mbox to use with my iBook G3 (600Mhz) which is in the list of unsupported computers in Digidesign's site.
http://www.digidesign.com/compato/osx/mbox/
How true is this?, does anybody run this configuration succesfully?
(I need to run a max of 6 tracks...)

I don't think you'll have much trouble w/your MBox and a 600Mhz iBook. If you are using ProTools LE 6 make sure you run either 10.3.2 or 10.3.4 since 10.3.3 was not completely tested and has got some problems with PTLE 6.4
As far as 6 simultaneous trax, well that depends.
If you plan on having 6 full tracks of recorded audio from the Mbox you may experience some lags, because of the plug-ins/effects - I have a 900Mhz G3 iBook and after about 6 tracks with EQ and reverb I start getting lags when/if I use ANY plug-ins at all. And you are going to want to use plug-ins. EQ, delay, reverb, Amplitube LE, Reason, Live, etc. -- all of these plug-ins and Rewire apps are goingt to tax your CPU and you may not get the kind of performance that you are hoping for. Still, a 600Mhz iBook is a good machine that will work great for single track recording and moderate editing. I would suggest that you consider buying a desktop Mac or PowerBook G4 for your more ambitous projects, but don't rule out your iBook for mobile recording and editing.

As far as Digidesign ruling out your computer, they don't - they are saying that they sugest at least 800Mhz G3 or faster - that doesn't mean that 600Mhz G3's won't work, just that they may not be optimal. If my 500Mhz iMac can run ProTools LE in OS X, then your 600Mhz iBook will. Just don't expect to make "Dark Side of the Moon" with it.

lushbudget