I just figured it out for my Droid Razr.
1. Click on Handcent SMS
2. Go to Settings
3. Click on Application Settings
4. Click Default Messaging Application
5. Select All(SMS & MMS)- That will make Handcent the "default texter"
1. Go to the "home page" on your phone
2. Go to your applications
3. Scroll through until you find the original Droid texting app
4. Touch and hold down on the app until it comes to your home page and drag it up to the top where it says "More Options"
5. Select App Info
6. On the page that it brings up click Disable. There will be a warning message, but just click ok. Then Handcent should now be the shortcut on your lock screen!
WORKED FOR ME!
HOPE THIS HELPS!!