It's kind of a late reply, but still...
Some of you asked the question: Who is the intended user of Roundcube? Who is going to install it on his server, or who is going to provide it as an installable on his webhosting platform?
To find out which programming language suits best, there are probably several ways to find out what your intended users want/need. One of them might simply be to look at what is supported most. http://w3techs.com/technologies/overview/programming_language/all
The link above will tell you the same thing as any other similar websites will tell you: "PHP is used everywhere, the rest of the languages don't seem to matter". What might also be of interest, is that most of the websites use PHP 5.x
Moving away from PHP might result in that the new RC will no longer be the number 1 choice. Demand for the "old" (the current) version might even stay higher. Is it already decided how (and if) the "old" version will remain supported? and for how long?
Furthermore, there is already a lot of good and reusable and thoroughly tested code in the current version. It would be a waste to recode it all.
I really like Node.JS, but I think it would be a poor choice.
Martijn Thie