Actually, I dint find the answer, but I can now exactly reproduce the problem. Try sending an email to a Gmail account from your Gmail account BUT USING another adress. When you open that email and press answer, Gmail will put your own adress in the "to" field instead of the sender.
Example.
Johnny has a Gmail account called VeryHardTOremember@gmail.com.
From this Gmail account BUT USING ANOTHER ADRESS (JohnnyATwork@work.com) he sends me an email at myadress@gmail.com
When I open my gmail account I found the email from Johnny:
From: "Johnny <JohnnyATwork@work.com>"
To: me
When I click on "Answer" gmail puts in the "TO" fields my own adress myadress@gmail.com instead of Johnny adress.
If Johnny sends me an email using the gmail adress instead of the other one everything is ok. Notice that this happens ONLY If answer the email from gmail.
I hope the explanation was clear.