I've spent a lot of time over the last year downloading and playing with applications for the iPad that have some relevance to geology. However the dream of using a lightweight tablet with a stylus and built-in GPS for field mapping is getting closer to reality. I always thought the limiting factor was the ability to make use of digitizing on the tablet to draw contacts, annotations, etc. However, I think the free iCMTGPS app provides a viable tool for actual field-mapping. It isn't as slick and easy as using ArcPad, however it works and it's as accurate as many of the GPS units are folks are using for 1:24,000 mapping.
You can email the support staff at CMT for a user guide, but it's a little thin on some of the specifics so I put together a short review and tutorial illustrating how it can be set up for field mapping applications. Strengths, weaknesses and quirks are all discussed in the video. Any and all feedback is appreciated!