Since EE is, at best, vapourware, all we have is the functional SDD documentation.
For CE, on the other hand, we have the original CE manual, plus its original design spec, and lots of miscellaneous bits and pieces.
I agree DocBook is probably the best answer overall; I haven't gotten round to getting my hands properly dirty with it yet, I just haven't had time.
On the subject of a Wiki, it's doable, however I think I'd want separate CE & EE wikis if we went down that route. If this is something people will think is beneficial and practical, I can roll out a wiki implementation (typically MediaWiki unless anyone has any burning issues with that? - only problem is that for now I'm limited to 1.6.10 due to not having PHP5 on the server yet, and some of the stuff on the server has not been upgraded to run cleanly on PHP5)
I just think that the wiki may help us get some progress on EE at least as it would so people where it stands.
You could run php 4 as a apache module and php 5 as a cgi binary until you are ready to switch to php 5 completly.
But if its too much hassle then leave it until your server is ready.
The guys doing it need to either make some "visible" progress or step down and let someone else take up the job..