global variable in number button (numeric field) in Blender 2.5


I’m new to blender, so please excuse my possibly trivial question.
I would like to have a set of parameters which can modify my model. Is it therefore possible to have a global variable in a number button? I thought about in-line python expressions, like for ipo drivers. I also saw an earlier post for an old Blender version, but I was not successful for 2.5.

Thanks a lot!