Search by IP address is available in ACS 5.3.
Couple comments if decide to upgrade to this release
- install patch 4 for 5.3 as part of the upgrade. This has key fixes for AD connectivity and upgrade issues
- I had seen it mentioned somewhere in a CDETS that search was not possible on devices that were upgraded and only on added/modified device definitions after the upgrade. If this proves to be true after upgrade should export the devices and then import them again and search should be possiible