I'm writing this in case anyone else runs into it.
I hadn't hooked our Epson Picturemate up to my printer for awhile, but when I did for the first time after upgrading to Leopard, I ended up with a 0.10" trailing white edge on my borderless pictures. I could even see it in the iPhoto Print tool - the image showed that there was whitespace on the right edge.
I tried a bunch of stuff and I finally got it to stop, although iPhoto still thinks its going to print white. In the Print dialogue, click the arrow so you get all of the options. Look at Paper Handling and check "Scale to fit paper size", and uncheck "Scale down only". This seems to work great.
The new Picturemate seems to have an option in the driver where you can tell it that it is misestimating the size of the paper - maybe that's what's going on here as well.
As an aside, I took this as a chance to try the demo of Adobe Lightroom. I was very impressed (and it didn't have this problem). Now I've got to decide if I want to pony up some $$ for it.