Latest update approx Tue Jan  6 13:00:48 PST 2015

Information about possible/probable group-buy of 4x4 button pads.  
First some info and photos and then some details.
Click on small photo to get full size photo.
All taken with phone so not great but should get the point across.

Side by side comparison of the two available heights of button pads.  Dimensions are the same except the height of the buttons is 13.7mm vs 8.7mm.  The shorter one will not fit a 5mm LED however it may just fit an SMD.  Not really interested in it as I plan to use a fairly thick front panel to make the mechanics easier.
Buttons from the back with a 5mm LED shoved in the hole to give you an idea on clearance.
Shot from underneath.  Button not depressed.  Plenty of clearance for a 5mm LED.
Button pressed while at the same time pushing the LED up.  Note the clearance between the LED and the board.
The pads are 100mm square.  My plan is to do something like 18x18.  Its gonna be big...  Sorry this photo is upside down.
The back of a pad, with rule.  I know I'm mixing imperial and metric here.  When you build things in the US, you mix them.  It usually works out unless you are NASA.
A shot of the back with rule and with backlighting.  The intention being to show the inside diameter of the buttons.  The holes look to be approx 9mm.

Some things I've noted. As you can see, they are intended to be able to be ripped apart to 2x2 button arrays. The samples I got, they rip apart VERY easily on the tear line. I think planning for this by appropriate holes for the nubs on the bottom of the pads is a really good idea. As far as LEDs. The inside of the button is approx 9mm. Maybe 8. Hard to throw calipers on something that stretches and its hard to eyeball, so lets assume 8mm for safety. It looks like a 5050 SMD LED will fit inside that with room for solder pads. Won't know for certain until I do a board layout. For certain a 3535 SMD LED will fit. In both cases you can get RGB LEDs with plenty of brightness. Of course you can also so 5mm RGB LEDs and they fit just fine, but wanted to mention the SMD options.

Plan at this point is to buy approx 100 of them. This would put the cost each at around $3 or so. I'm fuzzy on this because it depends on how many are ordered and the final shipping price. I think shipping DHL is excessive but that is what they have quoted at the moment. Shipping inside the US will be somewhere between $5 and $15 depending on how many are shipped. Shipping outside the US is probably cost prohibitive but I can ballpark it if you're interested. They are fairly heavy so priority mail flat rate boxes will be the order of the day where possible.

I have not put up a tech drawing of these. The one they supplied me was a badly cropped version of the Sparkfun drawing ( BUTTON_BASE_V0-1-3 ). I take this to mean that they are supplying Sparkfun... in any case you can grab the tech drawings off Sparkfun's site. Sparkfun sells their buttonpad for $10. For my project I need 20 of them so I'm already near the break-even point of a bulk order otherwise I'd just order from Sparkfun.

If you are interested, drop me an email xyzzy @ sysabend . org.