With the proper software, it is quite easy to recover most if not all the data off of a formatted drive. Off the top of my head, I can think of several pieces of software that would work quite well for that. OnTrack Data Recovery software, Norton has an un-format utility that works rather well, and I believe even Windows has (or at least had) an unformat utility that worked somewhat. The only way to absolutely ensure that the data is non-recoverable is to physically destroy the drive. Even doing a full data destruction where the drive is overwritten with 0's is not fail-safe, as there are recovery methods that read magnetic resonance on the drive. Hope this has been some help.
-J