Upgraded to Mac OS 10.7 (Lion)?  Tried to update Macports with sudo port selfupdate and get:

Error: /opt/local/bin/port: port selfupdate failed: Error installing new MacPorts base: shell command failed (see log for details)


You probably still have the Snow Leopard version of Xcode installed.  Try to run Xcode, it will probably tell you that you need to upgrade.  Go delete the Install Xcode app from your Applications folder, fire up the App Store app and get Xcode again.  That will get you the installer for Lion, which you can then run to upgrade Xcode to the Lion compatible version.  Then the Macport update should work.