Opera Mini, Gmail, and various Twitter applications are three of the most commonly requested icons to theme. Since these are all Java applications, it makes it impossible for the themer to alter their icons. However, there are various tutorials on the internet, such as this one, that explains how you can unpackage a java application to change the icon prior installation.
For those who wish to do this, here are the icons I have made:
Download The Icon Pack
Included are: Opera Mini, Gmail, and Twitter.
These icons are also great for those who wish to learn about icon making. Viewing the various shapes and layers will help you better understand the foundations of vectorized imagery.