I have tried this probably 6 or 7 different ways, such as using various attribute values, XPath, id pattern matching (it always matches ":\w\w"), etc. as locators, and nothing has worked. If anyone can give me a tested, confirmed-working locator string for this button, I'd be much obliged.
If you want to emulate a click on the button, just go to #compose.
How to access Gmail's "Send" button using Selenium RC for Java or C# or Python