I gave my form a try using opera (latest windows opera version, latest wordpress version, latest Gravity Forms version).
The signature box is displayed and you can move the cursor around as if signing, but it seems to me to be using invisible ink! Even though you can't see what your signing - the form passes validation for required field - so there it thinks something is there - you just cant see it.
(I incorrectly raised this in the ie9 signature bug post so please delete that one as it is a duplicate).