Obviously not. The thought is ridiculous. And most sane people would agree that a young child's choice of dress-up doesn't have much bearing on his or her future gender identity. And, if it does point to future proclivities (which it may or may not depending on the child)–is there anything to be gained from not letting a kid be his or herself?
Girl dress up clothes have a tendency to be more fun…glitter, colors, shiny shoes, makeup. Why should girls get to have all the fun?
I read a fabulous post on the Nerdy Apple Bottom blog detailing one woman's decision to let her 5 year old son wear his choice of costume on Halloween (see pic above) and the nasty reactions that ensued from other parents.