In response to those wondering about whether or not PB's ship with Tiger:
from
http://www.apple.com/powerbook/software.html -
"Every PowerBook G4 comes standard with Mac OS X Tiger, the worlds most advanced operating system."
So I hope that means if you buy it online, you're safe...
Additionally, I have been struggling with the same problem for months too (to buy or not to buy) and have finally decided that computers progress and change so fast that I will buy a powerbook now, and love not having to deal with the joys of windows. Once I've made excellent use of it, Mactels will be well developed and reliable (rev b / c) at which point i would normally update anyway. There will ALWAYS be a new update, but the best advice I've heard yet is that if it works well for your applications now, it will work just as well for them a few years down the line.
As for what updates are to come, I also believe the final update to the PB line before the switch (if there even is one) will be a minor speed update (1.8GHz) and MAYBE a standard 128 video card.
Just so you know, I'm not qualified to make such predictions what so ever.