Blake-
With Formatting Rules you can't dynamically set text on a label you can only change its apperance.
What you could do is stack 2 labels on each other and use Formatting rules to only show the appropriate label.
Something along the lines of:
Condition: [FieldInTable] = False
Visible: False
Also when you create a Formatting rule don't forget to actually apply it to a label, intially when you create one it doesn't automatically apply.
The other alternative is to use a vb script in the Scripts > BeforePrint event found in the property grid, though this is a bit more advanced.
-Michael