Sorry, I was in a bit of a rush because I had to eat dinner and hang out with my lady, but I forgot to address the issue of corporate taxes.
You asked: What kind of tax calculation is this? Which corporation pays 31% tax rate on earnings? Certainly not in the US! If this is what Panama charges corporations, then I would suggest that this is not a very pragmatic investment scenario.
This rate is actually pretty good given that the mean corporate income tax rate for 2006, after the inclusion of state taxes, was ~39.3% (see Wikipedia: http://en.wikipedia.org/wiki/Income_tax ). In fact, "the United States has the second-highest overall corporate income tax rate (39.3 percent combined federal and sub-federal) among all [Organization for Economic Cooperation and Development (OECD)] countries" (see: http://www.taxfoundation.org/publications/show/1466.html ). This means that, if the corporate income tax minimum was 35%, then the range would most likely be ~35-43%. However, looking at the financials of MSFT and WMT, it is apparent that their effective tax rates were ~30% and ~34%, respectively, most probably due to tax write offs. This suggests that the previous range would have been greater.