Apparently only dual link DVI will display 120hz. The full specs and user guide are available to download from the Samsung product page. The manual gives some good tech info. I guess I will let you know tomorrow when I pick my screen up.
Very little reviews and solid info is a little un-nerving for £600 but tomorrow will hopefully put the mind at rest!
The display port can run at 120Hz. If it couldn't then the S27A750D wouldn't be able to use it at all since it doesn't have a dual link DVI.