I have the fiskars I linked to below. It has a measurement guide on the fold out arm and it has the wire to tell exactly where it will cut.
I feel extremely stupid telling you all this, but since I have had this trimmer (January of 2010) I didn't know how to correctly use the fold out arm and measurements to cut paper until about 4 weeks ago........ I would use the 'ruler' on the edge to mark where I wanted to cut, then cut. Durrrr. Since I figured out how to use it correctly (it actually just came to me when I was sitting there trimming) it has made life so much easier! Hehehe