Arduino integration will be awesome, but to me, true IoT will be when we have some sort of low power compact mesh network support. Arduino mini is close, but once you add the RF transport, it gets bigger.
I'm envisioning something not much bigger than a CR2032 battery with a small microcontroller, a sensor, battery power, and a mesh transceiver. CC2530 looks pretty good and you can get boards from China with antenna for less than 10 bucks.
Plug one of them into a Pi serial port, and run some code behind to collect data, and boom, you just made a wireless low cost mesh network for pressure, temperature, noise monitoring, entry alarms, etc... and with Cayenne, it's on the net and pretty looking.
UPDATE: I think we've found it in the ESP8266. Search the forum for some great info... like this HowTo --> http://community.mydevices.com/t/esp8266/820/20?u=kreggly