When the need comes. They seems to do it when they have time and confident enough that the current release is stable enough. 5.1 has already gone through two .x updates (upto 5.1.2) showing it's not exactly ready for prime time systems.
That said, don't ever expect Java to be compiled into PHP. Two reasons:
One - It's highly unstable. There is no production quality Java/PHP bridge out todate.
Two - It requires persistant Java processes (uses 5 Java processes no way to limit it but increases as load increases).
If you want to play with it, I'd suggest to do what I've done and compile it yourself. One warning, you cannot use this permanently because the PHP/Java bridge requires dedicated persistant Java processes to be running for the "Java" conversion. DH doesn't like people running persistant processes. Which means while you'll get away with starting it up and playing with it; you'll need to shut them down when you're done "playing".
As for Tomcat, that's been discussed on the forums in the past. With the recent updates to Tomcat (year or so) there's been new thoughts about the possibility of running a Tomcat server. However, I hadn't herd one way or another since then.