Android - HTML signature in Gmail app

One current work around would be to create a draft in gmail that has the signature and use it in the mobile app.

There is an open ticket to the Gmail application team via the android project to address this. Please vote for the issue if you wish to see html signature support in the native gmail application.

http://code.google.com/p/android/issues/detail?id=17752

The gmail app does not support html signatures, like you noticed only the actual html code will be sent and displayed.

You may want to try K9 or TouchDown.