Eclipse sometimes is unable to start properly if its workbench directory resides on the network. There is a .metadata/.lock file in the workbench folder which Eclipse cannot remove. This is a Java-related bug which on some filesystems prevents locking.
Workaround: Add
to the command-line options for eclipse. If you are using the Mac OS X application, append this line to the initialization file Eclipse.app/Contents/MacOS/eclipse.ini.