Hi,
I just received some stacking SIP headers from Adafruit Shield stacking headers for Arduino (R3 Compatible) : ID 85 : $1.95 : Adafruit Industries, Unique & fun DIY electronics and kits
I used them to replace the SIP headers on my "base I/O shield".
I'm now able to stack 3 shields. Pretty soon I'll have a PC 
Works for me!
John