Works fine in Word 2007, too. Note, this is not a formfield property. It is a font formatting.

For years I've had forms that I've tried to have appear as if filled in on a typewriter. I've created a field character style and applied it to the fields as well as to a bit of space before and after a field. That puts the input in underlined Courier New font. I had no problems with this in Word 97, 2000, 2002, 2003 or 2010. (I used Word 2007 as little as possible.)

I don't know why it would make a difference, but in Word 2010 and 2007 I use the Lock button from Greg Maxey's Add-In.