Originally Posted by SoftWyer
It might be that the button has become associated with an application that is no longer installed? If a phone power off/on doesn't fix it then you could try re-installing the Google Search application from the market.
Actually installing from the market was a part of removing the original black themed market.
Removed it with TiBackup, uninstall voice search, reboot, install Google search form market, install voice search, reboot. They all work, except will not launch from soft button.