Well, AmigaOS itself is tied up in so many ownership disputes and original patent/licensing problems, it'll never be freely available.
Even if by some chance you managed to get ahold of the 3.1/3.5 source tree.... Ask Hyperion what kind of condition it was in when they recieved it. From what I understand, it was a complete mess. It took them a couple months to just sort through it and get a semi-reasonable CVS together. Tons of not-so-portable BCPL code, too.
Basically, what I'm getting at here is that a free AmigaOS of any version level isn't going to happen. Period.
Now, AROS.... Is basically exactly what you are wanting. An open-source re-implementation of the AmigaDOS system on much more portable code base. It's actually progressing quite well. Check out the AROS MAX 0.2.2 listed in the news section here. :-)
