Here is how I reproduce this bug:
1) Physical keyboard must not be open/visible, and screen needs to be in portrait mode (not landscape)
2) Have a texting thread open with someone (need to have already texted someone)
3) Click on the 'Type to compose' textbox for onscreen keyboard to popup
4) A few seconds later, click on the lock button on top of the Droid for the screen to go black
5) A few seconds later (time must be short) click on lock button again and slide to unlock
6) Onscreen keyboard will have disappeared, click on textbox
7) Onscreen keyboard will fail to be displayed, and texting app appears to be frozen
8) Click on back button on bottom of screen
9) After about 15 seconds, you will receive the dialog "Sorry! Activity Messaging (in application Messaging) is not responding."
10) You will have the option to click "Force close" or "Wait", clicking wait allows you to return to the app with the keyboard open ready to type.