Several iPhone users are experiencing visual feedback lags when attempting to type messages on their iPhones. Symptoms include key animations not activating when keys are touched. The issue does not appear to be for a specific letter or area of the keyboard and is generally considered intermittent, though many users report a consistent problem while typing in the SMS application.
Apple Support Discussions user Delapidator relays their experience with this issue:
"On the iPhone keyboard. You know when you touch a key on the keyboard and as you touch the letter it pops up but larger? Well, occasionally when I'm typing certain letters don't do it. The letter itself does go into the message body or whatever I'm typing into but it just doesn't pop up big to say that the letter has been typed. Not always the same letter, although certain letters do seem to suffer this problem more often than others. S and I are [the ones] that this happens to quite a bit. On the other hand sometimes they all work just fine?
I know it's not a really big problem coz at least the letters are working but sometimes if you're looking at the keyboard and not what your typing you think you haven't touched the letter and keep on touching it and it ends up looking a bit like ttttttthis! lol."
1. Be sure your iPhone is updated to the latest software--currently iPhone OS 2.2.1.
2. Reset your iPhone. Hold the top button and the home button until the iPhone turns off and the Apple logo appears.
3. Plug your iPhone into your computer and restore it. Be sure you back up all your data, as this will wipe your iPhone clean and restore it to its factory settings. When the iPhone restarts, choose to restore from your backup.
4. Restore and reboot as a new iPhone. Follow the same restore procedure as above, but when the iPhone reboots, choose to restore as a new iPhone. This will create a new backup--effectively deleting all your settings and some of the information on your iPhone. Of course, anything you have backed up on your computer (music, movies, contacts, calendars, etc.) you can restore to your iPhone. Text messages, statistics, and other settings will be lost.
5. Though highly unlikely in this type of situation, if the problems persist after completing the above procedures, your issues could be hardware related-- perhaps related to the touch screen located under the glass face of the iPhone. Take your iPhone into your local Apple Store or to an Authorized AppleCare Technician to have it checked out.
Apple Support Discussions thread describing iPhone keyboard lagging issues.