I wanted to use Eclipse (3.7) with Scala-IDE nightly (it's the recommended setup to work on Scalac) to work on SBT. So, I tried using sbteclipse for this!
I found so many issues (with sbteclipse) that I'd like to say that it just didn't work. If you ever want to tackle this, just try doing it again.
But I'll summarize what I did from memory.
I built SBT, used sbteclipse to generate projects, did the same for SBinary since Eclipse asked for that (that might not be needed any more), and then tried to import existing projects from the SBT directory. Since they are not all in the same folder (unlike scala/scala's src/eclipse), Eclipse found just a few, and I started painstakingly importing each of them. Code generation was still only done through SBT - I probably added the folder with the generated code to the source folders; and of course I needed to disable automatic building. I had to punt on one Precompiled_X project and remove it from dependencies.
My current Eclipse projects, if you are interested, are here:
See also #141. I believe that was the only truly blocking issue.
Regarding your other comments: