The Gemara in Makos (20b) says that a man is prohibited from pulling out white hairs. The Rishonim explain that the issur is based on לא ילבש גבר שמלת אשה, this is a feminine thing to do. This is brought down l'halacha in Yoreh Deah סימן קפ"ב סע' ו where the Shulchan Aruch paskens that a man cannot pull out white hairs and likewise he is not allowed to dye his hair. In the same סעיף the Shulchan Aruch paskens that a man is not allowed to look in the mirror.
The Ran in Avoda Zara comments that if the minhag is for men to look in the mirror then it is no longer a feminine activity and it is permitted. This Ran is the basis of the practice today that men use mirrors.
R' Moshe(יו"ד חלק ב סימן ס"א)addresses the issue of a man dyeing his hair for work reasons. He paskens that if a man dyes his hair solely to get a job (he thinks that if he comes with grey hair they will reject him as too old), then he is permitted as it is not done to beautify himself. The שרידי אש the לבוש מרדכי and R' Ovadya Yosef are also lenient in this case.
It would seem to me that nowadays dyeing hair has become like looking in the mirror. it is no longer an exclusively feminine activity and therefore in 2007 just like it is mutar to look in the mirror it may be mutar for a man to dye his hair.
