Since I’m really behind the notion of people moving to Python 3 (not only did I write a book about it, Python 3 is a better language), let me recap your options for installing Python 3 on Ubuntu systems. The same approach will work for Debian boxes. More here
As I mention in The Quick Python Book, the easiest and most reliable way to install Python is to use a precompiled package. If you’re using Karmic or later, that’s no problem. The packages are right there and ready to go. Go for the python3 and idle3 packages and you’ll get a complete Python 3.1 environment with no hassle at all.