Hi,
while working on http://bugzilla.slf4j.org/show_bug.cgi?id=170
I noticed that org.apache.log4j.Log4jLoggerFactory requires org.slf4j.helpers.Util
It should be added to log4j-over-slf4j's MANIFEST as an Import-Package:

The difference is at the very end of:
http://github.com/hmalphettes/slf4j/commit/2d30df3bd74e920d505308b322579d6fbbb4e1ab

log4j-over-slf4j/src/main/resources/META-INF/MANIFEST.MF
@@ -3,4 +3,4 @@ Bundle-SymbolicName: log4j.over.slf4j
Bundle-Name: log4j-over-slf4j
Export-Package: org.apache.log4j
-Import-Package: org.slf4j, org.slf4j.spi
+Import-Package: org.slf4j, org.slf4j.helpers, org.slf4j.sp

Thanks!
Hugues