To be honest, it was tried at a place I used to work at.
Its a tough one, they would have to be sure that no-one else could have physically had access to your computer at that time.
Also a good solicitor could ask for all the records of useage for the period they are trying to nab you about.
I would take it as a warning and stop surfing in company time.
Have to be honest, where I work now, I have a relaxed approach to my staff using the internet. Especially at lunch times or before/after work. We have some good filters on the network so i am happy they are not accessing something they shouldnt.
By making it so strict is not the best practice to be honest, its like making a personal phonecall, some people need to access the internet while at work, I would rather they did it and not took time off, if you see what I mean.