I guess I was lazy and used the logic exposed and used internally by SBT, which is SHA-1 based. :) But seriously, I don't think using a slower hash function will be noticable when considered against the work we're *not* doing by using the incremental API and when considered against the overhead of accessing files on disk. So I'd rather just go with what SBT does elsewhere unless it turns out to be a problem.

By the way, this stuff isn't visible in the main API so we could easily change the logic here at some point in the future if we needed to, e.g. to use MD5 hashes or to use modification times instead.

@jsuereth Thanks for taking a look. I was planning on asking you and Mark to take a look sometime soon anyway!