The Surface is a fairly sturdy device, and Microsoft has been pretty outspoken about how well it can stand up to being dropped, trodden on etc. There have been a few real-world examples already of people's Surface tablets being dropped and even run over by cars and still working perfectly (with maybe some cosmetic damage of course!).
That said, Microsoft certainly don't slap a guarentee on the product that it's indestructable, or indeed spill / waterproof. Few manufacturers (even Apple) accept items for free repair or replacement if the product has clearly been mistreated. Your best bet if you're concerned about accidental damage is to see if you can take out a home insurance policy that will cover accidental damage of your gadgets. If you plan to take your Surface outside of the home (it is a mobile device after all!) you may need to take out a separate gadget-insurance policy to cover you for such damage, and of course theft.
Extended warrantys may be false economy as some have pointed out - but with many of us carrying around at least one expensve gadget which could easily be damaged, or make us the target of crime, insurance is something I view as a very good idea.
