Well, I done this mod in March, and its still working perfectly. It is quite a major mod to do though. Firstly you will need a wheel which has quite a deep offset from the wheel rim to the boss. The DFGT has a flat offset. The wheel I used as the doner was a momo wheel from a Lotus Exige. It has a deep offset to accommodate the airbag, so any similar wheel would do. The diameter is 12 inches.

Removing the DFGT wheel is easy. flick the horn button off using a flat screwdriver, and there are 4 phillips screws holding the wheel to the base. When these are undone pull the wheel towards you gently, as there is a ribbon cable inside the column. Unclip the ribbon cable at the connector.
Thats the easy bit.
Now turn the wheel over and undo the back screws to release the front plate holding the circuit board. The back of the board in the wheel housing is pictured
HERE on Gran Turismo HITL site.
Thats the second easy part out the way.
Now its time to take a deep breath. In the pic on the HITL page, you need to remove the R1 and R2 micro switches and extend the wires to work the paddles on the new wheel. Cut the wires in the middle, and splice new wire in between using solder or bullet connectors.
Second deep breath part. using a hack saw, cut the original boss from the back of the DFGT. This will be placed inside the hub of the new wheel using nuts and bolts. When positioning it inside the new wheel, make sure its dead central. Attach it to the hub using nuts and bolts.(you may have to drill holes for these)
Then fix the front of the DFGT to the new wheel. You will have to drill through the spokes of the wheel in the same places as where the original screws held the front to the DFGT. When fixing into place ensure that the micro switches are pulled through into the area of the paddles. Glue the switches in place, so that they are activated when the paddles are pulled. Then screw the front into place onto the new wheel.
Re attach the new wheel to the base in the reverse order you removed it.
When you first go into GT, you will have to remap the gear change button in wheel setting to the L2 and R2 buttons, and if you done every thing right, happy days.
WARNING.....This mod is very difficult to do, and I had my reservations before I finally decided to do it. It is like an operation, and you do risk destroying your DFGT if you dont get it right, therefore I hold no responsibility over any wrecked wheels that may occur due to this being carried out. This should only be carried out if your brave, and are competent at soldering etc.
However, with the right wheel, and the right amount of prep and planning, it really is a superb mod to do. I only wish I had taken pictures of the process to share.