An article at securityfocus claims a single drive wipe is enough to prevent electron microscopes from recovering drive data.

"A computer forensics specialist has a message for security-minded computer users: A single wipe will make drives impossible to read.

In research published on Thursday, auditor Craig Wright tested the ability of a special type of electron microscope, known as a magnetic force microscope, to read data that has been erased. While overwriting the data multiple times with a random series of 0s and 1s makes it harder to recover, Wright found that it is nearly impossible to recover any meaningful amount of data after a only single pass. Recovering a single byte of data, for example, on a used drive is successful less than one percent of the time, he found. Accurately recovering four bytes, or 32 bits, of data only works nine times out of each million tries."

Read more: http://www.securityfocus.com/brief/888