I never had this problem with 1.1.1 on my G4 tower, but I see this fairly frequently with 1.3 on my MacPro.

I've got the CPU meter (set to average the 4 cores), and the network meter (set to display throughput with transmit/receive) in my menu. Occasionally I'll come back from a reboot and the network meter will be gone. Opening up the MenuMeters preference panel, I find that the checkbox on "display network menu meter" is missing. Checking it adds the meter back immediately, no problem.