Is MorphOS even planned for the G5?
Depends on how you define "planned".
Bigfoot did a basic port to G5-PowerMac earlier this year, and its probraly a safe bet that he hasn't moothballed it.
On the other side you have the team-statements that make it clear that they can't say anything about it and that there is not even a hint of an release date (just like with the the PowerBook-port).
So buying a G5 for MorphOS is stupid as it will take 2-200 years till it's gonna be released and you'll be able to get much better deal at that time...
