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I think the problem has to do with Microsoft Expression Media 2 not being able to handle file wrappers correctly, but I'm not 100% sure.

OmniPlan documents are stored in a type of folder called a file wrapper. File wrappers appear to be normal files in the Finder. Apple Mail will treat file wrappers like normal files and transparently zip them when you attach them to an email, but there are some applications that don't know how to handle file wrappers correctly like Microsoft Entourage. In these cases, the file wrapper should be zipped before being accessed by these applications. You can zip a file by right-clicking on it and selecting "Compress" from the context menu.