I successfully created a Google Toolbar button but did not submit it to Google becuase it requires a permanent hosting location and thought that it would be better to have it hosted by RTM. In order to submit it you would also need to add a "locale".

For anyone who uses Google Toolbar and would like to copy mine, copy the following text. Save it as C:\Documents and Settings\[username]\Local Settings\Application Data\Google\Custom Buttons\rememberthemilk.xml to add a button for RTM:

<?xml version="1.0" encoding="utf-8"?>
<custombuttons xmlns="http://toolbar.google.com/custombuttons/">

<button>
<site>http://www.rememberthemilk.com/</site>
<title>Remember The Milk</title>
<description>www.rememberthemilk.com</description>
<icon mode="base64" type="image/x-icon">AAABAAEAEBAQAAAAAAAoAQAAFgAAACgAAAAQAAAAIAAAAAEABAAAAAAAwAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAACAAACAAAAAgIAAgAAAAIAAgACAgAAAwMDAAICAgAAAAP8AAP8AAAD//wD/AAAA
/wD/AP//AAD///8A//+AiIiP////+H///3j///939/9/93///3fw/wj3f///d/////d///94////
93////j////3f///+P8HCPd////3fwgA93//93h/f3f3iIeIgIgACAgIeI+AAAAAAACIh4gIBQh4
d3/3d4eP93f/////eI/3iH/////4j/8I//8AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA</icon>
</button>
</custombuttons>