Hi Kazuharu,
Firstly, I mistakenly thought that VirtualBox marked odd sub versions as development. For some reason I had it in my head that VirtualBox 4.1 beta would become 4.2 GA. I've corrected this in the SVN repository so use branches/vbox-svn-4.1 instead please. I apologize for any inconvenience.
phpVirtualBox 4.1-0 will be released as soon as possible after VirtualBox 4.1.0. There is no set schedule. I imagine it will depend on how fast the translators translate it and if there are any more major changes to VirtualBox 4.1 before it is released. I usually translate all that I can myself - taking translations from the VirtualBox source code. This is still being worked on.
Let me know if you have any more questions at all.