I've seen the "Building..." bit as well on occasion. I agree that it seems to be a Mac issue, but haven't seen it often enough to conclude much more than that. Googling about now, I see others encountering this issue with no mention of OmniFocus. One comment made was that cleaning caches seemed to cause this, at least for some users, and while I hadn't made that connection, I think that the last time I recall encountering this, I had just cleared my caches with Applejack. Do you use Applejack, or Onyx, or any of those sorts of tools to clear out caches?
Here are two external threads discussing techniques that have worked:
http://discussions.apple.com/thread....readID=2569438
http://veritrope.com/tips/snow-leopa...-and-building/
No endorsement of those suggestions is meant or implied, use at your own risk. That said, the suggestions in the Apple discussion forum post seem like they should cause no lasting harm, which is harder to claim about 3rd party system maintenance utilities!