Office for Mac Excel app doesn't show thousand separator
A temporary workaround is always simply Command-A to select all cells and clicking this convenient button in the ribbon under the Home tab in the Number section:

A more permanent solution is as follows:
Go into System Preferences &gt; Language and Text and click the &quot;Region&quot; tab.  If the first region dropdown menu is United States (Computer), this might be your problem.  Looking below shows that this region formats numbers without a thousands separator.

This appears to be the result, as you stated, of an OS X update. Instead, set the Region to United States (without the Computer part).

From the Excel Help window:

Display or hide the thousands separator
Select the cells that you want to adjust. On the Home tab, under
Number, click Display the value of a cell with a thousands separator.
Note: Excel uses the Mac OS system separator for thousands. You can
specify a different system separator by changing the regional settings
in Mac OS X International system preferences.

We have just done that, but it appears that Excel continues to ignore the change.  Re-install Office and update us with the results.