According to Nat Tuck, Dec 2006
If done correctly with modern nuclear technology, Nuclear Power is a safe and clean energy source that can provide our electricity needs for thousands of years. It's our only real option to replace fossil fuels.
Thousands of years isn't forever - so it's not strictly "sustainable" - but given the potential of future research over that time, it's Sustainable Enough.
If you have any questions, comments, suggestions, arguments, flames, or other feedback, feel free to email me at nat@ferrus.net. I'll be putting some effort into improving this with any feedback I get.