A few people see the error "Network Libraries Unavailable" but it's not common and there's no well defined cause. In fact, the Palm OS 5 on the Treo 600 generally squelches most of the old style Palm OS "Fatal Exception" and "Fatal Alert" error messages by simply forcing an unannounced reset of the OS. The netlib error is one of the few old time errors you still get to see.
The Treo 600 has put itself into what is a bit like Windows Safe Mode: core functions are working but programming libraries for connecting to the internet via cell phone network or connecting to another handheld via IR have been knocked out.
The fix is nothing exotic... just be patient and perform anywhere from 1 to 4 soft resets. After each reset, try to connect to the internet by tapping on "Connect" in Prefs, Network. Sooner or later the Treo goes make to normal and can connect.
