Hi Andrew,
a CASE would not fit your requirement, because the CASE function supports only a simple case at the moment (only a = comparsion possible).
I tested your IF on a SPS10 system and it works w/o problems.
Did you check maybe the result with the data preview function (which limits the recoreds) which returned only records fitting to the "> 7 days" category?
Regards,
Florian