It is acceptable (but not required) to include the rainbow colors in the logo. Sometimes this will look like it's been done before, sometimes it might look great. You decide.
Please avoid being cliche.
Try not to submit logos containing the Greek symbols for male http://en.wikipedia.org/wiki/Male not win the contest. But if you come up with something super creative that does include this, you never know. Generally I find it to be tacky.