Weird problem when compiling with 2.9.0 or 2.9.1

Hi,
I recently tried a sneaky upgrade of our project from 2.8.1 to anything 2.9.*
I've cleaned down the entire project, and when I compile, I get this strange result:
> clean [success] Total time: 4 s, completed Sep 1, 2011 10:31:01 PM> compile[info] Compiling 117 Scala sources and 88 Java sources to /Users/rbarlow/Development/projects/myproject/build/classes/... [error] /Users/rbarlow/Development/projects/myproject/build/classes/(No such file or directory)[error] one error found[error] {file:/Users/rbarlow/Development/projects/myproject/}myproject/compile: Compilation failed [error] Total time: 21 s, completed Sep 1, 2011 10:31:28 PM> exit
$ ls -al /Users/rbarlow/Development/projects/structuringlab/build/classes/total 0 drwxr-xr-x  3 rbarlow  staff  102  1 Sep 22:31 .drwxr-xr-x  6 rbarlow  staff  204  1 Sep 22:31 ..

So, the directory definitely exists. The above example is using sbt, but I get the same result if I compile in IntelliJ (to a different directory).
Everything works fine in 2.8.1.
Has anyone seen anything similar?
/Raymond