EDIT - 01.12.2015
There was an error in the Y-clamps. Can't believe I didn't notice, but the new ones are fixed.
These two parts let you extend the Y-axis to 280 mm. The Y-limit is a part that lets you mount the Y limit switch farther back on the printer, and the Y-clamp is a replacement belt clamp that moves the clamp point towards the motor, and clears the tensioner pulley when it's far forward. The U-shape at one end is there to grip the edge of the Y-carriage to improve stability. There is one for stock bearings and one for my lightweight bearings ( http://www.thingiverse.com/thing:1149490 ) which build 4mm. It fits T2.5 belts. Let me know if you need one for GT2.
I am using them on my printer, and have ditched the hotbed for a 250x300 mm acrylic plate to print on instead as I only print PLA. You need to change the firmware to increase the available bed size as EEPROM is locked in the stock configuration. This is the firmware I used as a base: https://github.com/rene-aguirre/Marlin