I'd say yes. Apple's entire user base uses PPC...they're not going to leave their entire customer base high and dry. I expect the PPC to be supported for years to come. Of course, that assumes you want to run Mac OSX. Linux is well supported on most Mac hardware. Personally, OSX bugs me. Perhaps it's the fact that I don't know how to use it, but I find that I really can't DO anything when it's put in front of me. Something as simple as raising a window is a challenge.
Actually, I'm waiting for the Intel macs to come out for exactly that reason...I want to grab a cheap PPC laptop! The hardcore fans will be jettisoning their old G3s and G4s in favour of the new x86-based laptops, so the time will be perfect to pick one up in the sub-$1000 range.
Like I said before, I'm not too concerned about OSX. Maybe I'll dual boot and give it a shot for a while, but its claim to fame is eye candy and usability. Well my friend and I sat OSX down side by side with Gnome 2.10 a while back, and wouldn't you know it, Gnome came out on top a good number of times. When we were finished, my friend had changed his mind about free desktops..."right on par with OSX" were his words. And as for eye candy, customize xcompmgr correctly and you have drop shadows that look identical to the Mac. Linux can now do all the cool 3d accelerated stuff that OSX does, and I prefer Clearlooks GTK engine to Mac's Aqua (never did go for the bubbly or brushed-metal look). The only thing I wish I had from OSX is Adium ( http://adium.sourceforge.net/screenshots.php sooooo sexy ).
While Gnome is essentially a replacement for OSX as far as I'm concerned, what Apple does offer is slick hardware. A standard iBook will go hours on end on a single battery charge, looks slick, has a tiny form factor, and pretty decent specs. The moment people start selling their iBooks, I dump my compaq presario desktop replacement juggernaut of a laptop and buy myself a smooth silent little second-hand deal from apple. Can't wait!