[whpalmer4]

As long as you don't edit the same file simultaneously on more than one machine, you shouldn't have any issues with Dropbox and OmniPlan documents. I think if you do change the same file in two places, Dropbox will catch that and bring it to your attention for resolution, but it would (potentially) be a pain to extract your non-trivial changes from one file and apply them to the other, and I wouldn't care to promise that it would always notice this problem.