http://neil.fraser.name/news/2006/11/17/
Use the tool found on that website. Set X Y and Z to 1, if you want a block to be the centre, check 'Centred on block' for each axis. If you want the centre of the sphere to be a corner, leave them unchecked. This creates a blueprint for a solid sphere, if you want a hollow sphere, you need to work out yourself which blocks to leave out (it's not hard).
My mod idea. It's about introducing genetics into minecraft.
Use the tool found on that website. Set X Y and Z to 1, if you want a block to be the centre, check 'Centred on block' for each axis. If you want the centre of the sphere to be a corner, leave them unchecked. This creates a blueprint for a solid sphere, if you want a hollow sphere, you need to work out yourself which blocks to leave out (it's not hard).
Go there. There is a diagram to help you, and at the bottom are two links to very useful tools.
Enjoy.
EDIT: The above poster's method would also work =D