The 2.5.1 update is recommended for all Application Loader users and contains enhancements and general fixes including:

Improved error messaging
Creating and sending In-App Purchase details for apps directly from Application Loader
Send to Apple command now creates logs for troubleshooting delivery issues
Updated system requirements and newly supported formats
Updated server IP addresses and port information
More detailed information on this update can be found in the Application Loader guide available at itunesconnect.apple.com.