It's far too early to say what the significance of the two recent chemical weapons finds is. While it's certainly true that one shell does not an arsenal make, one counter-example definitely does make a refutation.

If the sarin shell is just an accidental leftover from previously-known arsenals, then it is not evidence of anything interesting. At the other extreme, if it is of a type that it has hitherto been believed Iraq did not have, as has been suggested, then it is significant because it proves the existence of a secret chemical weapons programme after the time when Iraq persuaded the UN that it had ended all such programmes. And the only possible reason for a new chemical weapons programme under those circumstances would have been an intention to produce chemical weapons.

As Rachel Ann says, all this remains to be determined.

If it turns out that there was indeed a class of chemical weapons being produced in Iraq of which the US had no inkling, then the CIA, FBI and US government have made a terrible mistake and should all resign. (Just kidding.)