Info here: http://www.google.com/glm/gmail
Point your phone’s browser (assuming you have some kind of mobile internet plan on it) at http://m.gmail.com to use it.
It fails to work on my Nokia 6800, it thinks cookies aren’t enabled on my phone’s browser, but they are.
Dagnabbit, this would really be useful for me if it worked. I reported the error and gave them a detailed description, maybe it’ll be fixed soon.