I've had this a happen on eBay a few times.
Did you have a Reserve price on the camera?
I think it works like this:
If you have a reserve price set for the item, then the Buy It Now will remain until the bids exceed the reserve ... after that it will not show.
If however you have no reserve price set for the item, then the Buy It Now will disappear as soon as the first bid is placed.
It's definitely stupid logic on their part ... and they should atleast warn you of this before you list your item.