It's all very nice to assume we're all friends and since there are four admins, the chance of them inappropriately censoring posts or removing users is probably small. :)* The administrators reserve the right to remove any inappropriate post or user without any notice.
However, it does *sound* rather authoritarian.
i understand the legal threats - governments and corporations in the USA, China, Russia, Poland, France, Burma, Kenya, you name it, are afraid of distribution of information beyond their control on the internet and can put lots of pressure using scare tactics to close down web sites, including in universities. All the same, i don't see any government or corporation wanting to close down a cosmology forum...
So IMHO it would make more sense to say that admins reserve the right to shift any post to a "hidden" category. The hidden category would still be visible, but you can hopefully add something to the BBphp scripts to insert something like <del><strike> here is the censored text </del></strike> plus some standard disclaimer. You can see an example on the polish indymedia (open news publishing, unrelated to cosmology) site: http://pl.indymedia.org/pl/2005/02/11762.shtml (no, it's not phpBB, sorry).
Both del and strike are used because of changes in html definitions; not all browsers respect both, but together they should function in most browsers AFAIK.
This way it's obvious that the material is considered unacceptable, but people who want to "watch the watchers" or "guard the guardians" can at least check and decide if the hiding of the material is valid.
The fundamental problem is that it's difficult to have rational debate about censorship when the material itself is unavailable. Third parties have to play a game of A said that B said that C said that ... which gets very difficult and boring to disentangle. Instead, having "hidden" material, maybe better described as "obscured", is a step towards solving this: most users are not upset by the material, since they don't read the hidden section, and claims of censorship can be easily judged on their merits by third parties who make the special effort of checking the actual content.