I tried this on the Geoserver 1.3.0-beta2 version and this
query works fine for me.You didnt say which version of geoserver you were using - I
expect you were using a 1.2.x version. Why dont you check
out the 1.3 version?
This is fixed with resolution of jira:GEOS-328. Not related to 'Not'
filter at all, but the depth of logic.
John