In Maya 2011 perspective view Dolly (Alt+MMB)has been occasionally sticking (stopped dollying) and has now stopped working altogether. The other Alt functions work and the MMB works in other functions.
Thanks Ben, while doing further testing and searching based on your suggestion I have stumbled across the cause of my problem; it is my mouse button. It is only working if rocked sideways while pressed: must be dirty contacts or something.
Thanks for your interest.