Old Pencil build
Win7 Ultimate SP1 64bits
I thought it would be of some information to mention this.
The build I used was not the last from @Matt since there is still some work to do with the importing and exporting Movies. So I am taking as a reference the tests that I made using the 0.5 beta-old-code (18Mar2011).
I was not able to save more than 600 frames. It crashes after that.
When Exporting to Movie, Pencil was not able to render all the frames already presented on the timeline. I had exactly 700, but the final render showed only 566 frames.
When the export and import features were fixed I will have a look to these same tests