I'm sure I am missing something, but why the impetus to trade Lidge now? Loretta/Lamb appear to be a capable 3B, Wheeler is closing all right, the team is winning, the Astros are already on the hook for a significant percentage of Lidge's '07 salary, and his improvement dramatically strengthens the Astros bullpen at this point.
If you must, at least hold onto him until the deadline, when some desperate contender may be willing to part with a young Bagwell type from the system. You can keep Scott Rolen and his kind.