I am using Helios on Mac Snow Leopard. I don\'t know why but all of a sudden my arrow keys and delete button start not working only on Eclipse (so Eclipse ignores them) but
In Windows and Linux environments I have experienced such issues when the system believes a modifier key is pressed. By pressing and releasing all modifier keys the issues often perish. However, on OSX I did not experience such issues, so this might be entirely wrong.
If the simple idea does not solve the issue, you could try to start a new workspace to see whether some preference causes the issue; or you could download a new Helios instance to check whether its working. I know, these are all serious issues, but may locate the problem.
Otherwise, you shall take a look at the question asked not long ago about keyboard issues, maybe there is some hint how to solve it.