Cathy Wong at About.com has already done an exemplary job of summarizing a study indicating that vitamin C helps your body absorb more antioxidants from your green tea, so I won't reinvent the wheel.
I will point out, however, that this study is one in a long line of research pointing to this complementarity between green tea and citrus fruits.
I would also be remiss if didn't remind the unfamiliar that there exists an age-old tradition of adding a piece of, say, dried orange or mikan peel to a pot of tea.
Yet more evidence that the ancient ones knew just what they were doing.
Don't throw that away.