According to Microsoft manager David Gainer, the issue is related to changes made in the to the Excel calculation logic:

Specifically, Excel incorrectly displays the result of a calculation in 12 very specific cases (outlined below). The key here is that the issue is actually not in the calculation itself (the result of the calculation stored in Excel’s memory is correct), but only in the result that is shown in the sheet. Said another way, =850*77.1 will display an incorrect value, but if you then multiply the result by 2, you will get the correct answer (i.e. if A1 contains “=850*77.1”, and A2 contains “=A1*2”, A2 will return the correct answer of 131,070).”

Apparently Microsoft already has a fix for this issue and will become available for download as soon as it passes all the required tests.

As David Gainer stressed out, the flaw only exists in Excel 2007 and previous versions are not affected.