Hey everyone - after my Epson RX640 stopped printing and trying all the usual fixes etc (with no joy) I read the MacFixIt article here and although the particular fix recommended there didn't work for me it did point me in the right direction to discover my own fix.
It seems to work well and has helped a few others but please try at your own risk if you're not completely sure what you're doing.
Baiscally open the Console utility,
Click 'show log list' if there's no drop down menus at the left,
Then under the drop down menu 'FILES' you should find 'system.log' (click on it) - try to print and look at the latest info.
It should give some idea of what isn't working correctly and in my case showed which file was causing the issue.
For me it's a file named 'IraMW1' - Apple's Epson printer software update 2.7 seems to install a faulty version of this file.
If this is the file that's causing you issues too it's a pretty easy fix if you have a backup from before Apple's Epson printer software update 2.7 (my IraMW1 file that works is dated 3rd December 2009)
To find the file I went to: /Library/Printers/EPSON/InkjetPrinter2/MW/IraMW1.framework/Versions/A/IraMW1
Then simply replaced the new faulty IraMW1 file with the old working one from my backup.
= Printer working again!
My process in a little more detail here
Hope it helps!