I have tried this and it sure works. Rename the alert log and have a new alert.log file so that oracle can start writing to the blank file. I guess oracle p'bly would create a new one but to be in the safe side just have a blank file out there.
If you run Linux, do not try to rename or move trace files of background processes, you will not crush Oracle, but Oracle will keep file handle opened even if file does not exists anymore. In that situation all trace information will not be written, new trace files will not be created.