Ok - You've only got my word that this is a valid tip, but I have been reliably informed that Sainsburys are sitting on 10,000 Wii's ready for the Christmas explosion - Sorry I can't give more details but it sort of comes from the horses mouth.
This makes me wonder how many other large chains are also sitting on stock.
I can't validate this or offer any more info but as the price on Ebay etc. seems static at £270 - 300 (for basic sports pack) it might be worth sitting tight for a while then if necessary going for the crazy ebay prices.
Anyway I'm just trying to help so please don't shoot me down in flames, I'm a mum myself I wouldn't be so cruel as to hoax (or actually find the time!!).