As you may know, several months ago, I created an Adobe Photoshop template for creating comps that work with Olav Bjorkoy's Blueprint CSS Framework. Today I'm also adding an Illustrator template to my Resources section and I have updated the Photoshop template to include horizontal guides.
Both templates are simply documents with guides based on the 24 units provided by Blueprint. The background image of the grid is also provided with a layer to show unit numbers if you like, and the Illustrator document shows a fake comp to demonstrate positioning on the grid.
You may download the templates here:
I welcome your feedback.