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Check out this whitepaper's section on secure deletion (pg 7):
http://www.sans.org/reading_room/whitepapers/incident/631.php
It should give you an idea of what "secure" really means, as well as a summary
of the DoD 5220.22M spec on what the government considers the minimum steps to
sanitize the data.
It also discusses the other issues that have been brought up in this thread
throughout the rest of the paper.
Keep also in mind that a a dedicated cracker may have the resources necessary to
counteract your methods. This is why one of the DoD options for purging data is
incinerating the drives. That said, that initial step you mention -- defeating
undelete software/requiring a cracker with access to proper equipment -- is
certainly achievable.