I’ve always gotten Safeware Laptop Insurance for all of my notebooks, it covers stuff the normal warranty doesn’t like theft and accidental damage. Depending on your insurance company, you may be able to add insurance for your computer through your homeowner’s policy, which might be a better deal.
For comparison, Safeware is about $100 a year, and it’s definitely been worth it for me. A careless coworker spilled his coffee into my PowerBook last year and I had a new one within two weeks. Later that month, my dog ran into the power cord and dragged the computer off the table, denting the corner and mangling the power input jack. Again, it was covered and saved me quite a bit of money.
I would never buy an extended warranty or coverage for my desktop, but since my laptops lead tougher lives I think some kind of protection is worth it.