Hi gdbanks, thanks for replying. I thought so too at one point but from what I can tell the correct nickname for the Ponderosa is actually the 5 Pound Lemon, because that's what it can usually weigh up to. It seems it gets incorrectly labelled as the 9 pound lemon.
The 9 Pound Lemon on the other hand does weigh up to 9 pounds and is a monster citrus. The Epcot Centre at Disney has them in their experimental greenhouses. Short passage about that here:
http://www.wdwnews.com/ViewImage.aspx?ImageID=101828
Unfortunately I can't find any detailed information about this variety, this is the only hit on google I get except for the mislabellings of the Ponderosa.
I found 2 youtube videos of the Disney Epcot centre that briefly show the 9 Pounder Lemon on their boat ride:
(watch from time index 3:39 to 3:56)
http://www.youtube.com/watch?v=Ipy1tF5DU-Y
http://www.youtube.com/watch?v=E1erptyD4RU
The first time I heard of the 9 Pound Lemon was in the catalogue of The Citrus Centre UK (page 9):
http://www.citruscentre.co.uk/The%20Citrus%20Centre.pdf
They specialise in selling only citrus and they list the 9 Pounder seperately from the Ponderosa which they also carry. Unfortunaetly they don't have any for sale at the moment, otherwise I'd snap one up!