GMAC may offer programs that allow you to terminate the lease early without any penalities if you purchase another GM car. Call them and ask.
You may also be able to sell your contract at swapalease.com and leasetrader.com.
But I am not too knowledgeable about GM Smart Buy... it is structured like a lease though... small monthly payments and then a large balloon payment at the end or simply walk away by paying the fee. According to GMAC, you can also sell your GM Smart Buy and keep the profits if you have any equity. But I doubt any GM car does in these times.... so you may be better off paying the $300 fee and walking away.