I will be as loyal to a company as they are to me. In the current corporate environment of looking at letting people go, trimming their benefits, etc. as the FIRST way to cut costs, if I had a better offer, and it fit my needs, I would leave.
For example, I happen to know a former executive of a well known computer manufacturor that had just completed the most successful, profitable program ever in North America. After it was done, did he get a big bonus, a thank you? No, they let him go. Thank you very much.
You may work at a great company, but I will tell you this. If they get into financial trouble, one of the first things they will do is look at who they can lay off. Or force to take a pay cut, demote.
IBM once required extreme loyalty to the company, even requesting that everyone dress in dark blue suits. Now how many people have they let go over the last 5 years. A lot.
Enjoy the good jobs, but don't think for a minute that most companies will be loyal to their employees. At one time yes, but not today.