...but instead seeing a grayed out "Erase Free Space" in Disk Utility?
Note: With OS X Lion and an SSD drive, Secure Erase and Erasing Free Space are not available in Disk Utility. These options are not needed for an SSD drive because a standard erase makes it difficult to recover data from an SSD. For more security, consider turning on FileVault 2 encryption when you start using the SSD drive.
Also google "erasing space on ssd".
Bottom line, it is not something you want to do to a ssd.