For a Project Manager, the difficulty in hopping from fundamentally different projects would be in executing project planning. The inventory of necessary materials, equipment, and specialists, time management, and overall project execution for skyscraper construction are completely different from those for software development.

In addition, as other posts have mentioned, some projects many require the PM to have a strong knowledge of certain relevant regulations and quality standards (e.g. FDA, OSHA) or even hold certifications, these would require training to obtain.

I agree that other qualifications of a PM are to have great administrative and interpersonal skills and that these are easily transferable between different projects; however, I believe such skills are much more easy to pick up then, for example, understanding the nuances of C++.

With the above mentioned, a great and versatile PM would be one with an interdisciplinary background; one of the reasons I chose Biomedical Engineering was because it can offer such a background.

References: PMBOK Guide