I did the Land/Sea at the Poly in April 04. Prior to the vacation I did alot of research on the subject (although I wished that I had done so before commiting to the Land/Sea).
If the previous poster got concierge rooms in was a fluke. This may have been during some of the recent renovations that the Poly has gone through.
Concierge is not available for Land/Sea clients in advance, even if they want to pay extra. If there is availability when you check in, you would have the option of upgrading at an additional charge. I do not have exact figures, but believe its roughly $120-$150 per night extra.
Land/Sea vacationers get Garden View. Should you be using the Disney Transfers, you would only be put in a few long houses. I was in Tahiti.
I was actually very impressed with my Garden View Rooms. I could not see any of the parking lots. It was a very quick walk to the transportation center monorail station.
While at the Poly, you should try the character breakfast at Ohanas. The food was very, very good (and expensive).