OWC released a do-it-yourself upgrade kit for 2011 27-inch iMac owners that'll let them add an SSD to their current hard drive. The US$45 kit includes all the tools and the power cable needed to add the solid state drive (sold separately).
The process requires you to remove the 27-inch glass display with suction cups, so it's not for the faint of heart. There's an installation video, which you can watch before you make your purchase. If the DIY is too complicated, you can still use OWC's turnkey service which will install the SSDs for you.