Always check out the original article at http://www.oraclequirks.com for latest comments, fixes and updates.
A quick note for those who came across a weird bug in Oracle SQL Developer that makes its editor virtually useless under Mac OS X.
If you are wondering why suddenly the backspace key doesn't work or the arrow keys simply do nothing, it seems that, for some reason currently being investigated, the keyboard preset for Mac OS X is not correctly applied or it's not properly loaded.
If you go to the menu TOOLS/PREFERENCES/ACCELERATORS, then you click on the button Load Preset, and finally you choose "default Mac OS X", you should be able to get the keys to work again.
And thanks to -K- for the useful workaround!
See the OTN thread where this problem is discussed.
Apparently this elusive bug affects SQL Developer both 1.5.1, 1.5.3 and possibly 1.5.4 according to Sue's reply.