The other day I stumbled upon Decimail [1], a webmail client that uses a client-side IMAP connection, tunneled through http.
This seems like the holy grail in responsiveness and desktop-like feeling for a webmail client. Could roundcube ever adopt such a model? It would be a total rewrite, since you wouldn't use PHP anymore... but then again, the GUI stays exactly the same, and that's the part about Decimail which really sucks.
[1] http://decimail.org/index.html