Go to settings, then applications. Click on "manage applications" and then bring up the menu to see the "filter." Filter so that you see "all." Scroll down to "home" and click it.
When you do that, you should see a "launch by default" that says it's set to launch by default with certain situations. Click "Clear defaults." Click your home button, and you should be able to re-set Open Home as your default. Hope this helps.