According to a Slashdot post, just about all python code will require at least some changes when python 3.0 comes out in early 2008. I've never learned python, and news like this makes me glad that I've never bothered. (Besides, a syntax predicated on whitespace just seems weird to me.)
I wonder how Red Hat feels about this, considering that a lot of the RHEL system scripts are in python. They'll probably have a lot of rewriting to do for RHEL6.
I suppose that similar criticism could be leveled at Perl6 v. Perl5 (although there is talk of some sort of compatibility mode as well as a Perl5-to-Perl6 translater). But it's probably a moot point: as far as I can tell, Perl6 will never, ever be released.