Using FileMaker Pro 11 with Windows XP SP3, I am unable to replicate this problem.
In Form View, I have a white background, and a field without any border specified. When I tab outside the field, the borders are lightly dashed, and if I click outside all fields and commit the record, the field returns to no borders.
I've tried to replicate this on five different files.
Do you have a file you can send me along with the exact steps to replicate the problem?
Let me do some more checking on several different machines first, then I'll send you a copy. It might be a video card issue if you can't replicate it. I can confirm this happens with brand new test files.
There really aren't any steps more exact than what I've already posted.