A couple years ago I decided to learn Python. I took a quick look, saw Python 3 was the "latest", and started working on my app, using http://docs.python.org/py3k/ as my guide.

It wasn't until a month later when I went to deploy that I discovered Python 3 wasn't what everyone was using. It was a small app, so it only took me a a day to port it to 2.6.

My point is go ahead and learn Python 3. The hurdles of back-porting are pretty minor when you don't have any version specific habits.