Just to change the flavour a bit, I'd like to mention the human factor in cracking, as even the tightest, most secure site is wide open for inter-personal attacks unless ALL the users are well-trained on security protocols. You never want to hear "Gee it sounded like an IT guy, why shouldn't I have given him my password" or similar. Such old-time cons still work well, are cross-platform portable and immune to any techno defenses.

My standard plug: read the Risks Digest for plenty of horror stories!

Update: For an interesting though quite dated collection of cracking stories try Out Of The Inner Circle by Bill Landreth.

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I'd like to be able to assign to an luser