Nexus One: How to Clear Google Talk’s Chat History
Contributed by Rob Rogers 
Tagged: Android The Talk application on the Nexus One keeps a chat history of your conversations. While this can be useful, at some point it will need to be cleared out.
1. Open the Talk application.
2. Select the contact that you whose conversation history you wish to clear.
3. Select the Menu key.
4. Select More.
5. Select Clear chat history.
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Kumaran said on March 21, 2011
TO CLEAR GTALK CHAT HISTORY IN SONY XPERIA X10 MOBILE USE THE FOLLOWING STEPS. IT REALLY WORKS
Settings>Applications>Manage Applications>Gmail Storage>Clear Data