Developerworks are running a comprehensive series on Python version 3, and the changes that it brings with it.
Part one looks at print() - which is now a function, not a statement, and therefore requires parentheses. This, on its own, is going to cause a large number of pre-existing Python programs to break. It also covers the renaming of raw_input() to input, the new bytes literal, changes to string formatting and new I/O mechanisms.
Part two covers class decorators, metaclasses, abstract base classes, changes to exception handling and the handling of integer literals.
If you do any sort of Python programming, then you'd do well to read these two articles, as Python 3 is here now, and you're likely to be using it in the near future.