I would like to see the numbers for August and July 2008 compared with 2009 as I think enemy activity and CF/ANSF deaths are up in general and not specific to the winter. I was in the Helmand and Farah provinces so I can only speak to those areas. From what I saw all the big timers take off to see their families or take a break during the winter and some of the local players keep some level of activity up.
Historical statistics here OEF Fatalities

Jul-Aug 2008 = 31 US Deaths

Total CF for those months = 76

Historically CF
Jan-Feb 02 = 22
Jan-Feb 03 = 11
Jan-Feb 04 = 13
Jan-Feb 05 = 4
Jan-Feb 06 = 18
Jan-Feb 07 = 20
Jan-Feb 08 = 21
Jan-Feb 09 = 48

All the data is at the above linked site.