Anyone who has read my "Topic of the Week" series that I wrote over 2003/2004 knows this is a pet peave of mine. When ATC or another communication agency instructs a pilot to change frequency, it really bothers me when a pilot doesn't acknowledge. It happened several times at work last night. All that is needed is a simple, "See ya," or something to let me know you heard me. If I don't hear that, I have to say it again. Then again. Then I have to call the other guy and see if you checked in. Some have asked, "Why? What does it matter?" Well, unless you'd like a Dash 8 or a Cessna as a hood ornament, I have to know that you heard the frequency change and are with the right controller so if he has traffic for you, he can get a hold of you. It's awfully tough to provide ATC services to aircraft you can't communicate with. I've had several situations where I've called an aircraft three or four times, only to find out he was actually still with me, and was otherwise occupied in the cockpit. Imagine if I had just assumed he received the frequency change. Eventually he drifts out of VHF radio range and now the workload in trying to get a hold of him quadruples. I have to try to relay through other aircraft, maybe call company and get them to try contacting him. It's a little thing that can turn to a big thing totally unnecessarily. Just say, "Bubbye!" or something, and I'll know you are where you should be.