I've noticed this on my Uncle's iMac (2010). Whenever you put the computer to sleep, regardless of if you're connected via Wi-Fi or Ethernet, you get disconnected in Sleep mode. Then once you wake the computer from sleep there's a bit of a wait for your computer to get re-connected to the network. For instance, if I unsleep this iMac, then immediately go to Safari and try to pull up a web page, I get a 'blablabla you are not connected to the Internet' Safari window. Or, if I wanted to pull a file off another networked drive, I get something like 'networked drive not available'. Any ideas on how to sleep a Mac running Lion without it shutting off it's networking capabilities?