The evil nemesis of backups is data loss.  Lots of data loss comes from accidents, but sometimes it is the result of malicious acts.
Twitter was recently totally hacked (TechCrunch has a great article).  Hundreds of confidential documents were stolen and posted.  Users’ emails were posted.  The actual admin interface of twitter itself was hacked.  In this case, the hacker, “Hacker Croll” really did not do a lot of damage.  There are problems, of course, mostly coming from scrambling by the twitter folks.  But Croll could have done a lot more bad than he did.

Here are some important points:
1.  Back your stuff up in a different location…that way, if you get hacked and they deface or delete data, you still have it.
2. Practice safe computing.  Use good passwords, etc.  Twitter’s password was…”password” – To quote (from memory, so it could be wrong) SpaceBalls, “What kind of idiot uses 12345 as their combination”

Meanwhile, there is a security hole in the iTunes store that displays full credit card information, which is not PCI compliant and is against VISA and Mastercard policies.  Croll leveraged this while hacking the heck out of twitter, but it could be happening to a lot of unware, less famous individuals even as I type.