I've seen this asked a million times before, and actually, the solution is quite simple. You just use jQuery to get the Roundcube login form, modify it just a little bit and then add it to your own page.
luxusalusasut
business opportunity
I've written a small tutorial on how to do this, it can be found on
www.culturecrew.be/en/creative-code/roundcube-login-page/.
This approach works so long as the Roundcube installation resides on the same (sub-)domain as your page, because otherwise it would be considered cross-site scripting by your browser.