Ben its your anti alias setting or you could use layer mask when you create you artwork.
I ran into the same problem a while back and found a site that showed me how to fix it but cant remeber the addy anymore.
It might have been here
http://graphicssoft.about.com/od/pho..._Photoshop.htm
I work in either 20x30 print size or 4500res @ 300dpi so when i resize it really doesnt matter. if the lines are jagged
Zee