For years I had lucrative work performing wireless penetration testing for companies (it's important to note that I never did any penetration testing without a signed letter of authorization in my possession). WEP can be cracked very easily by pre-teens. It can be done tracelessly if you are willing to spend some time at it, and it can be done within about ninety seconds if you're willing to do some traffic generation.

WPA is crackable as well, but it's tougher. If you use WPA-PSK it behooves you to use a very long (20+ characters) unguessable key consisting of letters, numbers and symbols. WPA using cryptographic certificates is very tough to crack, but is beyond the capability of most (if not all) consumer wireless gear.