Ambiguous

Offer good until 3/3/09 or until supplies last.

What does this mean? That the supplies are guaranteed to last until 3/3/09, or that the supplies could run out at any time before or after 3/3/09, or that the offer is not good after 3/3/09 even if supplies last, but that the supplies could run out before that?

The offer is good until 3/3/09, but may end sooner if they run out of supplies before then. Meaning, no, supplies are NOT guranteed to last until 3/3/09.

The offer will still end on 3/3/09 even if there are still supplies left.

<Offer good until 3/3/09 or until supplies last.

What does this mean? That the supplies are guaranteed to last until 3/3/09, or that the supplies could run out at any time before or after 3/3/09, or that the offer is not good after 3/3/09 even if supplies last, but that the supplies could run out before that? >

Whoever wrote this made a mistake. It should read "while supplies last."

See Uriel's comment for the meaning.

It should be "offer valid until 3/3/09, while supplies last" although the original wording is a fairly common mistake.

Whether you think the offer is good or not is a matter of opinion.

Well, they are using "good" in the sense of "to make good on a promise" or "his word is good"-- i. e., to live up to it, or to stand behind it. Not "good" as in the sense of a value judgment (good vs. bad). So good in this sense does mean valid, and is not really a mistake.