Security is not just an afterthought, it's something you'll have to keep in mind constantly while working on phpBB. You can't start looking at security problems during the beta phase, you'll have to keep things tight from the ground up. So IMO from a security POV you should be fine. 2.0.11 is probably safer because it had quite a bit more testing but 2.1 should be good enough for most purposes as long as you stay proactive in keeping your site up to date!
If something serious is discovered in 2.1 then we are probably going to fix that pretty quickly because we are running 2.1 on area51 (and on our own development servers ofcourse). We're not giving any guarantees here, mind you
I'm just stating what is probably going to happen if a serious issue is discovered.If you are (very?) knowledgeable about PHP and databases and if you want to invest the time and effort then it should be possible to run a site on phpBB 2.1. We really don't recommend doing that and you still won't get any decent support but you can ofcourse do it on your own responseability.
