Unfortunately the 7931 phone will not be supported in UCM until 6.0(1). The 7931 was developed for CME to start since they are a true key system replacement, however we could have used 7931 support in Call Manager 4.1. :)
Here are the release notes on the 7931 only supporting Call Manager 6.0
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_ipphon/english/ipp7931/relnotes/31831.htm#wp1325634
Cisco Unified IP Phone 7931G Support
Cisco Unified Communications Manager release 6.0 and later supports Cisco Unified IP Phone 7931G. The Cisco Unified IP Phone 7931G is designed to meet the needs of businesses with moderate telephone traffic and specific call requirements. The Cisco Unified IP Phone 7931G supports IEEE 802.3af Power over Ethernet, security, and other calling features. Dedicated hold, redial, and transfer keys facilitate call handling. Illuminated mute and speakerphone keys give a clear indication of speaker status.
For more information, see the following documentation at http://www.cisco.com/en/US/products/sw/voicesw/tsd_products_support_category_home.
? Cisco Unified IP Phone 7931G Installation Guide
?Cisco Unified IP Phone 7931G Phone Guide
?Cisco Unified IP Phone 7931G Administration Guide