Well, here it is. A T-slot library for OpenSCAD.
size=10, //size of each side
length=10, //length. descriptive enough, no?
thickness=3, //thickness of the 'sheet'
gap=0, //gap, thickness of the lower part of the 'T'
center=false, //somewhat vague. todo.
nut=false, //set to true to make a fitting T-slot nut
There's also a bunch of presets, which only take length and nut.
Currently there's only tslot20, tslot30 and tslot30_3060.
http://www.8020.net/T-Slot-5.asp lists the parameters for each size.