Excel 2007 Calculation Bug
Question: When does 850 * 77.1 not equal 65,535?
Answer: When you're using Excel 2007.
A bug in Excel 2007 (and Excel Services) is causing issues around calculated values of 65,535 and 65,536. Calculations that evaluate to the values may incorrectly display 100,000 as the result (although internally, Excel stores the value as the correct result). Microsoft is officially calling the bug a "display" bug (see the link to David Gainer below) rather than a calculation bug. To the average end-user, it doesn't really matter what you call it because this boils down to a trust issue. As in, I trust that Excel always shows the correct results of calculations. The "don't use it until after SP1" crowd is collectively saying "I told you so".
This bug does not affect prior versions of Excel.
Like Simon, I'm a little surprised this issue hasn't gotten more attention. I thought I'd wake up today and see it on eWeek, Seattle P-I, etc. I guess everyone is still busy playing Halo 3. Anyway, here are some good sources of information on this issue: