I believe that emerge -p should never log anything, but when I give the command: emerge -fp package I get a line in /var/log/emerge.log like: 1152000000: *** Finished. Cleaning up...
This has been fixed in or before svn r3734 which was released in 2.1.1_pre2.
Actually, this was never fixed. Its in svn r4068 for 2.1.1 and r4069 for 2.1-r2.
This has been released in 2.1-r1.