Lion: Switching an Application's launch preference off of Rosetta (PPC)
Application launch preferences are stored in ~/Library/Preferences/com.apple.LaunchServices.plist, including whether or not to open using Rosetta.

Opening up this file (easier to parse if you have Property List Editor installed from the developer tools) will allow you to find the entry for the specific app that's set to open in Rosetta, and you can strip out the PPC-relevant bits. Make a back up first, just in case.

More info is available here.