But it is not unethical in the same way as fabrication or falsification of data is. Someone indulging in the latter is likely to get fired. Immediately. Ask Jan Hendrik Schön.
While not citing other people's work (particularly for interesting research ideas) may not get a person fired, it could still blow up -- particularly when this offence is repeated -- to create an international scale embarrassment to the culprit and his/her organization. Ask Armando Córdova.
Thanks to Janet Stemwedel for the link to the C&EN story by William G. Schulz.