If the $50 USD price tag for Super Mario Galaxy 2 is keeping you from pre-ordering, then maybe $20 USD to put toward a future game purchase will help change your mind:
Order Super Mario Galaxy 2 and get $20 off a future video game purchase. The promotional code will be placed directly into your Amazon.com account within 2 business days after the product ships. Offer valid when shipped and sold by Amazon.com. Promotion valid on all existing orders. Limit of both one per household and one per customer. Amazon reserves the right to change or terminate this promotion at any time.