I have been testing various environments for nav 2009. here are the results

nav 2009 with sql 2008 r2 and windows 2008 r2 dont work.

nav 2009 with sql 2008 and windows 2008 r2 works. For this to work you must install fixpack from microsoft before you install sql 2008. and after you install sql 2008 you must install sql 2008 sp1.

nav 2009 with sql 2008 r2 and windows 2008 works

nav 2009 with sql 2008 on windows 2008 works