REQUIRES ROOT. OPEN SOURCE: Source here https://github.com/grennis/GTalkEnabler

This application will enable Google Talk Video Chat over 3G/4G. With this app you will not be restricted to wifi-only video calls with Google Talk.

You must have Video-Chat enabled Google Talk installed for this to do anything at all (It comes with Android 2.3.4 but can be loaded on Android 2.3.3).

This app will display a simple settings screen that allows you to enable or disable the restriction that Google Talk Video Chat will only work on WIFI networks. With this application and root priveleges you can control this restriction.

This application works by toggling the setting gtalk_vc_wifi_only in the database gservices.db.