I've printed hundreds of 12x12 borderless layouts on the R1800 from PSE with no problems - maybe I can help? When you go to print, the first thing you see is the Print Preview from PS. Make sure your layout is actually sized to 12x12, and the page is scaled to 100%. Then click on the "Print" button at the bottom of the window. You may get a "Print Clipping Warning". Do NOT click on "Scale to Fit Media" - just click ok. Now the Print window will come up. Click on Properties. Choose Photo (we've tried photo and best photo - best photo takes about 45 minutes to print vs. about 2 minutes for photo, and I can't notice much of a difference at all). Under "Paper Options" you should have the following:
Source: Sheet;
Type: Enhanced Matte (assuming you are using the Matte Scrapbook photo paper from Epson);
Size: 12x12. Then click on the
borderless box (click ok when the warning comes up). Now click on the next tab at the top - Page Layout. Change the expansion from Max to either Mid, or the halfway mark (borderless will scale up your layout to ensure that you print all the way to the edges, if you leave it on max, you may cut off some of the detail around the edges of your layout - I only notice this if I have "inking" around the edges, or something similar). Now click OK and OK again.
Hopefully this helps!!

It is a fabulous printer! Please let me know if it's not working, and I can send you some print screens so you can see exactly what it should look like.