I can confirm that the calendar does not show in Word 2007 and does in Word 2010. Keep in mind that these two programs are not the same.

When removed from the text box, the calendar is visible in both.

It is possible that the "text box" is a shape first introduced in Word 2010 rather than an actual text box. I do not know. (Text boxes have been part of Word for a long time.) I agree that there is no good reason I can conceive to put a table inside a text box. However, I tried it in Word 2010 and opened the result in Word 2007 and it worked fine.