I also have this problem, but it's not just for SMS messages. Any time I type where the live wallpaper is visible behind the keyboard it happens. For example, I have multiple widgets that take text input.
It happened this morning while using the facebook widget to try to send a status update. I was able to duplicate it multiple times with different live wallpapers. The factor was 31 or 32 keystrokes. Not characters, but keystrokes. Hitting the shift key or the "?123" symbol key counted even though they did not put anything in the text field. Typing 16 characters, then backspacing to delete them even causes it. Just hitting the symbol key to hop back and forth between symbols and letters without typing anything will cause it on the 32nd stroke.
I tried it with the phone's search button, which leaves a tiny sliver of homescreen visible between the search box (suggested results) and the keyboard. it again happened on the 32nd keystroke.
If I just type the same letter over and over again, it happens on the 32nd one. if I retype the original message I was trying (which contained and at sign (@), a colon and spaces) it happens on the 31st keystroke.
I used ATK to free some memory to test if it might be a memory issue, but it still happened. I restarted the phone, and it still happened.
It seems to me that the issue is either the keyboard or wallpaper. Can anyone else duplicate these results?
I have a Motorola Droid through Verizon. Firmware 2.1-update1, build ESE81. I use Helixlauncher, which may be a contributing factor to the wallpaper display.