While it is absolutely right for an employee to always be on the lookout for a better career situation, it is wrong for other employers to try and pry people away through poaching methods. Gaining employees should be through normal means, it should never be in a targetted fashion while an employee is working for a competitor.
No, I believe employee poaching is ethical. I believe that it drives businesses to become better employers in order to compete for better employees.
If an employer provides a good work enviromnent, fair pay, and adequate employee benefits they would not have to worry about losing their employees to a better company.