Stacking PCBs with High i/o.

divice the required connection into current carrying and signal carrying.

The current carriers will be few.

Then create some zebra strip patterns on the joining surfaces.

look up zebra strips here.

http://www.google.ca/search?hl=en&q=%22zebra+strip%22&btnG=Google+Search

What they are is a layered connector with condiuctive rubber and insulating rubber laid down as strips into a block, which is then cut to make your connector. The thickness of the conductor can be called 1 zebra width.The pads are separated by 2-4 zebra widths and a section on each side is compressed into contact. As long as the alignment is good, a strip cannot 'stray' and connect wrongly.

There are companies who make and sell these systems, contact them

Take a look at the link below

http://www.hypertronics.com/printed-circuit-boards.html#

They have contact used in testing BGA's that may be referred to as "fuzz buttons". There are lots of contacts in a small area.

Good luck

You might find some of the products from Samtec of interest.

http://www.samtec.com/technical_specifications/overview.aspx?series=GFZ

for a suggested solution.