This is a so weird bug in Oracle SQL Developer 1.5.1.
I am a fan of this application for my work.
And some day keys did not work. For example, left, right, backspace, delete keys did not work at all.
Nevertheless, on any other applications keys were fine.
There is no perfect solution yet, but reset will fix this problem.
Go to Preference -> Load Preset -> Default-> Ok.
It will be back then!