It seems that the biggest contributors of Plan 9 are no longer in the project and it does not seem that there is a big development. There is 9fans, a mailing list for Plan 9 users, and once a year they usually do a meeting, but I have been surfing the source code and there's a lot of code from 2002-03, other from 2005-06 and a little from 2008-09 (http://plan9.bell-labs.com/sources/plan9/sys/src/).
I wanted to give it a try as a daily use OS (as I am an student and do not need anything serious right now) and I have no idea whether to use it or not because of the development it's going on.
Thank you. It would be great if someone who is up to date with Plan 9 could give me an answer.
I have contacted with one of the organizers of the Plan 9 meeting and conferences and the development is active, although not with a lot of people, but it's interesting for sure.
Here it goes all the development of the system based on Plan 9. Programmers from Bell Labs, for what the person I contacted told me, are also involved in it, so check it out.
This mister also is the lead developer of Plan B, a set of user programs which run on top of Plan 9 (and, I guess, of Nix-OS as well, which is based on Plan 9).
They have even ported it to the Nintendo DS, so if you are one of those that has a Nintendo DS in your home, I'm sure it would be really interesting to check out all the projects related with Plan 9. Here it is the Inferno OS for Nintendo DS (based on Plan 9) that runs on NDS. InfernoOS is also a variant of Plan 9 and runs in any PC (it's not the same as the one I have just talked about. It's the one the Inferno OS NDS is based from).
Also, if you are in Linux, you can run it from Linux itself without a Virtual Machine. Just download the ISO of Plan 9 (or variants, like Nix-OS) and install 9vx. More info, here.
Hope this helps. If I find more information, I will for sure update this post. Sorry for the mess of information.