You are correct that the "hard" part is the sensors in the grand scheme of things. With iPad accessories, Apple has very stringent terms for developers who want to utilize the dock connector to interface with external hardware. This is why so many items rely on using WiFi or bluetooth in order to communicate with an iPad, iPhone, etc. I had seen your previous comment about how Rutan flew his plane with info from a Mac that communicated over serial and that is what spawned my comment about the particular adapter I had found. In terms of data acquisition, the sensors, signal conditioning, and shielding are the main areas that can make or break data quality. I'm not the most knowledgeable about avionics but I don't see much of anything that I don't feel a homebuilder could adequately replicate with a small embedded system such as an Arduino. Once that was setup, the data could be transferred back to the iPad which has all the processing power you would need. I can't imagine the sampling rates being anything special so it would be pretty awesome to have the functionality of a very expensive flight display at the price point of an iPad plus some accessories and development time. No, it wouldn't be certified or anything, but you have to start somewhere and isn't that what experimental aviation is all about? The iMonitor and associated modules from TruTrak that zk-jkw mentioned are a step in the right direction as far as I'm concerned.
