This came about when trying to reduce the size of my time machine backups. Most of the things I wanted to exclude were in /library but in finder it has mysteriously vanished. So here's how you get your Library Folder back in Lion's finder.
Launch Terminal from Spotlight or Launchpad -> Utilities, and enter the following command to show or hide the directory:
chflags nohidden ~/Library/
he users Library folder will immediately become visible again. Reverting this back to the standard Lion setting is simple too: Hide User ~/Library in OS X Lion (default setting)
This returns to the default setting of hiding the user Library directory:
chflags hidden ~/Library
Changes take effect immediately again, and Library becomes invisible to the user.