While this is not currently supported, what I have used to achieve this result was to create a label in the form and enter my description there. Not 100% fun, but it allows me to customize the font as for coloring, bold, etc...and place it wherever I want.
Hope that helps some.
While this may not be what you need, I've found that the column description actually shows up as a tooltip to the control in Nintex Form. When you hover over a control in the nintex form it will display the tooltip value equal to your column description.