There is the IMS (snow and ice) 4 km data available from NSIDC in GIS format (geotiff). I downloaded an image from day 275 (2006) - sometime in September when all lakes should be ice free. When you display the image all open water areas (lakes on land) have a value of 1. It is in a polar stereographic projection. This is the best product I have found thus far showing lakes. As I said before I have 1 km data, but for Canada only.