As helinus said, there is no way to get rid of this at the moment.
You will see this if you use the gmail Accounts -> Send email as option to add another email address.
What's happening here is that Google are setting your email "From:" header to the address you specify, but they also add a "Sender:" header, and set it to your gmail address.
Some clients will show it fine, but others - most notably, Microsoft Outlook - will automatically show the "on bahalf of".
This will be resolved only if gmail provides an option to remove the Sender: header, or replace it with X-Sender:.
There is no "security" issue here, since they already verify when you add the email address that you own it. Oh well. Perhaps they'd like everyone know you are using gmail - free publicity never hurts :)
More info on the Google Mail support area - How do I customize the 'From:' address on outgoing mail?