I bet we'll see a Leopard Special Event for it's release, that will either announce it's shipping that day or within a few weeks.
Special Events, with few exceptions like NAB, have given little more than a week lead time beforehand. If Leopard were to have a ship date announced of 2 weeks from the S.E., that would give at least 3 weeks from the date of the original invite.
We will have Leopard by WWDC, I think we're all in agreement about that.
At the latest, assuming Leopard is launched at WWDC, 3 weeks would give a late May announcement.
Now, I'd think Apple would try to give developers at least a week or two to have it before WWDC. Ideally a month or two!!
3 weeks plus 2 weeks lead time means the invitation arrives at the latest a week after NAB.
So we'll know within a week of NAB? I hope that wasn't too confusing

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But I really hope this article is true and we know by the end of March.
-=|Mgkwho