The iPad 3G has a GPS chip in it that apparently works well with
ForeFlight Mobile HD. Here is a
video demonstrating the iPad 3G by ForeFlight's cofounder and principle developer.
Specific to the thread question, there are a few
NMEA 0183 apps for the iPhone and iPad, such as the $11.99 NMEAremote that can send data an autopilot. You'll note when you click the "more" link in the Description Section of the
NMEAremote webpage, it specifies that you can "connect your iPad to all your NMEA devices over WLAN with a NMEA RS232/WiFi converter such as
iMux,
iAIS, Seamate 1A LITE," a USB/Bluetooth dongle or an RS232/Bluetooth adapter, depending on the autopilot. Nearly all recent popular aviation autopilots are NMEA capable. Several websites address connecting it to marine autopilots.
For example, the
TruTrak Digitrak User Guide, says to "Consult your GPS manual for NMEA output setup. The autopilot must have a direct connection with the handheld GPS to provide the autopilot with RS-232" and suggests you might install a "9 pin D subminature connector in your panel." Then an adapter as simple as the $3
Cablestobuy USB 2.0 to 9 Pin RS232 Serial Convert Adapter might be all you need to power and transfer data from the iPad.
While the iPad is not certified for IFR flight, it would certainly be an inexpensive GPS and flight information assistant under IFR, or could couple with the autopilot on VFR flights for experimental aircraft.