Anyone ever see an Eames Aluminum Group table with a leaf?  

Could be an after market top, or custom order perhaps
Post pictures and I can try to help.

Here you go. Emailed the...
Here you go. Emailed the seller, but have not heard back. I'm leaning towards it being something that was cobbled together, but never hurts to fish for an expert.
http://i42.photobucket.com/albums/e337/zoomob/Screenshot2012-11-03at1138...

Mix & Match
The top appears to have a birch edge like the top of a Nelson designed table, not the black rubber edge commonly seen on the Eames table.
The Nelson table was offered in white laminate and walnut and was offered in an expanding version with a leaf. The white laminate usually had a birch edge.
Since Herman Miller made both tables, it's entirely possible that someone put a Nelson top on an Eames base (either an employee, a customer, or even Herman Miller to meet a customer's needs).
I've never seen an aluminum group table with an expanding top. They were mostly sold for contract use and in an assortment of sizes, but with solid tops. There were very many produced so they are not hard to come by, and hence, not hard to find or expensive to buy.
I'd look at the table and if it looks well made, likely original to the base (and set of chairs) and meets your needs I would not hesitate to buy it.
If the top is in fact the variety you would find on a Nelson table, I do not think it significantly effects the value of the table positively or negatively. If it works for you, buy it and enjoy it.