The recent update to Apple's "Bonjour" zero-configuration networking technology has caused a few problems for some people. The update was to address connectivity issues between iTunes and the AppleTV, but a few people have noticed failures in a number of other network services after applying the update.
The problems do not seem to be related, and have ranged from the system failing to connect to wireless networks and other services, to the system date resetting itself. Additionally, for some the system has started prompting for passwords each time when connecting to network services.
It is unclear whether these problems stem directly from the update, but given many are network related and happened after a network technology update, it is possible the Bonjour update contributed to them. People have tried restarting their systems, resetting PRAM, and changing firewall settings to tackle the problem, but have not had much luck with these approaches.
Fixes that have worked have been resetting the network router, and also repairing keychains. Locate your router and power-cycle it by shutting it down and unplugging it for a few minutes. Then plug it back in and allow it to boot again. You may also try renewing your DHCP lease from the router, by going to the network system preferences, selecting your interface (Airport or Ethernet) and in the TCP/IP tab in the "Advanced" section click the "Renew DHCP Lease" button.
Beyond network settings, repair your keychains by opening "Keychain Access", selecting a keychain, and choosing "Keychain First Aid" from the "Keychain Access" menu. Enter your administrator password and click "Start" to verify (or choose the option to repair), and then repeat this with your next keychain (do this for at least the login and system keychains).