You probably know that "secure delete" built in to Snow Leopard is supposed to write over your data multiple times to theoretically prevent the data from ever being recovered.
Apparently the algorithms used on SSDs (solid-state drives) to make them look and operate like a hard drive are different then what the secure delete software is expecting to find.
Zdnet has all of the details:
SSDs, Mac OS X and data security don’t mix. Not only does the standard file delete not delete files - which savvy readers already know, but even the Mac’s “Secure Erase Trash” function leaves 2/3rds of a file recoverable.