I share your concern; it's not only annoying when something like this happens in a public place (such as in a restaurant), but if it happens during a church service it's both disruptive and disrespectful.
I'm sure those who forgot to turn off their cell phones didn't intend to disrupt the service; they probably would say they were just thoughtless. But that's one reason why I wanted to reprint your letter, because I'm afraid this often is our problem when we come to church. It's easy to approach a church service casually or even thoughtlessly; we come distracted and preoccupied with other things. As a result, we don't give our full attention to worshiping God and hearing His Word.
Take time (even the night before) to prepare your heart and mind before you go to church. Ask God to speak to you, and to help you to focus on Him during the service. The Bible says, "Be still, and know that I am God" (Psalm 46:10). It also says, "Ascribe to the Lord the glory due his name; worship the Lord in the splendor of his holiness" (Psalm 29:2).
God wants us to grow spiritually–and this happens as we learn from God's Word, and pray, and have fellowship with other believers. Thank God for the freedom we have to come together in worship, and ask Him to draw you closer to Himself every week.