I have never seen wood remain flat unless forced to do so; daily and seasonal humidity changes invariably cause warping of some sort and if the table is rectangular (rather than triangular) the slightest cupping will cause one bearing to lift off of the track... and there goes any hope of stability.
I posted an image in your Corian thread a while back as a bit of a joke, of a simple bearing truck that allows rail support while preventing uplift. The truck has also been made of Al and works well...there is a recently posted pic of that too somewhere here.
Not trying to push a bearing design on you, just saying it is so cheap and easy to make an assembly that prevents uplift I cannot imagine not building it into the system unless the table has to be easily removable.
And GuruNet says it is mouses
