Here's a good example:
http://i.imgur.com/zpjPl.pngI added the white lines in Photoshop - with acceleration on, the "Reports in September 2010" gets both narrower AND bolder, resulting in a very squished, very ugly presentation. The calendar headers are likewise far less clear.
http://i.imgur.com/gVFYk.png is from Basecamp - note the vast difference in navigation width, and the width of the "Comment" in the blue box.
http://i.imgur.com/LgGOJ.png is the Basecamp nav item blown up - the box on top is accelerated. It's thicker, more narrow, and it's got all those crazy colored artifacts.