How can I keep ~/Library visible in (Mountain) Lion

If you don't want to add a login item, you could also save this as ~/Library/LaunchAgents/com.stackexchange.apple.66594.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC -//Apple Computer//DTD PLIST 1.0//EN
http://www.apple.com/DTDs/PropertyList-1.0.dtd>
<plist version="1.0">
<dict>
    <key>EnableGlobbing</key>
    <true/>
    <key>Label</key>
    <string>com.stackexchange.apple.66594</string>
    <key>ProgramArguments</key>
    <array>
        <string>chflags</string>
        <string>nohidden</string>
        <string>~/Library/</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
</dict>
</plist>

You can use Automator to create a simple app using the Run Shell Script action (with the chflags command within) and put it in your Login Items (in the Users & Groups preferences). It will run every time you log in, so while OS updates will still re-hide the Library, you'll never notice.

Daniel Jalkut of Red Sweater Software has a pre-made app for this if you don't want to bother making your own. You can find it on his blog. Just download it and drag it to your Login Items.

No. Apple is firm in its decision to hide that folder. The only workaround is to either use the Go menu entry in Finder (simply hold down the OPTION key) or create an alias in ~/.bash_profile:

alias see='chflags nohidden ~/Library'

Then you only need to fire up Terminal and type "see."