Sorry for not replying sooner, I got carried away with things.

Thanks to afaq for identifying what the problem is and thanks to tiivonen for coming up with the solution. I installed fAPN and set up a connection called vodafone internet GPRS (I googled the details, but I can post them if anyone needs them). Youtube defaults to the desktop site now and works great! Thank you.

EDIT: @Meph1s: I have no idea why it would happen over wifi. Unless your wifi is connected via wap to the net, or you have bookmarked the mobile site.