With the default netmount script installed by Gentoo the dependency with Portmap is not correctly resolved. Therefor, portmap is not started before netmount. When this happens netmount won't mount nfs shares. I found out that there was an error in the depend part of the netmount script. There's an if statement there which contains "local mydeps="${myneed} portmap"" this should be "local myneed". After I changed it works correctly.
update your baselayout and/or run etc-update *** This bug has been marked as a duplicate of 7803 ***
*** Bug 10488 has been marked as a duplicate of this bug. ***