Let’s say I had an existing product that used an AVR ATmega64A. Let’s say that chip was in a TQFP package (splayed out leads) and soldered to the product’s PCB. Now I want to hack about with said product. (Specific example can be found at Google Code project gruvin9x – http://code.google.com/p/gruvin9x/)
What if there were unused pins I wanted to make use of – and pins I wanted to change the function of, involving some external re-wiring.
The attached image shows what I’ve had to do without any Spark Fun magic. Now …
What would be cool would be if I could de-solder the stock chip, throw it away and replace it with a socket. Then I’d love to have a Spark Fun break-out PCB with at ATmega64/128/whatever AND a plug on the under-side that slots into the new socket on the original PCB. Following me so far? 
Now I can readily access all the pins on the MCU, routing the original wiring to the original locations and re-wiring those I want to do new stuff with. There could be a convenient ICP connector on the board already of course – just like your existing break-out board product.
Can it be done? I found at http://www.winslowadaptics.com/ that they have a solder-down board that can replace the original TQFP component and break-out to 64 pins in a compact multi-dimensioned array. See here http://www.winslowadaptics.com/product/14679. Good start!
That product itself plus a nifty SparkFun PCB to plug on top of it could well do the trick. But I’m sure you geniuses could work out something much better using just the solder-down part off the bottom and do the rest yourselves, no? (If it’s available separately.) And I’m pretty sure there are “plugs” to go in sockets as I described above. That same company does have PLCC plugs like that. But I don’t think there are any PLCC sockets that will readily solder to existing TQFP pads.
Cheers!