File > Save As
Choose Adobe Ilustrator in the first window.
Choose Adobe Illustrator CS3 in the second window.
Be aware objects utilizing newer features such as gradients on strokes and guassian blurs in CS6 will be expanded when back-saving.
Here's a screenshot of the dialoge box you'll get after selecting Adobe Illustrator in the Save As window. Note that there are several versions of Illustrator that you can save your file as, but like Scott mentioned, there will be some aspects of your image that may be altered in the process of backsaving.