Since when are women allowed to wear shorts?

They can and they do - Venus Williams for example wore a pair of very short shorts at the US Open last year (2007) and in the first roudn of the Australian Open this year (2008), though i think she hasn't worn it in the subsequent rounds (perhaps it has something to do with the fact that one cameraman decided to focus on her butt during the first round game, and i think it was shown on the big screen or on tv)