As a DM, I've never yet allowed a player to play crossgender charcters. With an inexperienced player, I fear it would only become a distraction from the game and none of the more experienced players have shown the least interest in it.
What was really cool though, was when a party of first-time players (two guys, three girls) found their first cursed item: A belt of sex-revearsal! The female bard claimed it; she had been really polite and modest, letting the other players have every cool thing they found, but now she'd decided it was her turn to get a magic item!
Up until then she had really played the sexual charisma card, she had both the male players and even one of the female ones eating out of her hand, doing anything she wanted them to! Not that she'd had sex with any of them, she'd simply led them on with hints of all the exquisite pleasures she could grant them if they pleased her, and they were all competing to win her favors...
The scream of frustration (IRL) when she tried on her first magic item ever (this was still only her third night of playing!) was almost enough to make me feel sorry for her. She soon recovered however, and made the best of it. As I said, the players were all newbies and they had no idea how to get rid of the curse, so she got stuck playing a woman in a mans body for several sessions. She was starting to think she'd have to be a man forever, when she finally discovered a little spell called 'Remove Curse'.
She roleplayed it brilliantly the hole time and it only added to the game, never distracted from it. Even so, I wouldn't let a player make cross-gender character, nor would I wish to play one myself. I find it more than hard enough to portray female characters when I DM. But that newbie girl almost changed my mind, she delivered the finest roleplaying I've ever seen! Seriously!