No wallet is 100% secure except an offline wallet where you do the hashing on an abacus... Its entirely possible (likely) a 0-day exploit will be discovered and you will lose every bitcoin handled by your client...
No wallet can ever be completely secure. You can encrypt the hell out of it but the physical media you have it stored on can always become damaged. This is a great how to on making a decently secure savings wallet though. Very good information here. I will admit that I don't have enough bitcoins to even have a savings wallet yet but if I had any substantial amount of coins I would absolutely create a really secure wallet.
