It was just a week ago that I was sitting in a client's office, being asked: "...do you think BIM will ever be possible on an iPad?" Now the answer is yes! Here is one, of what I image may be many mobile device based BIM applications: RedStickCad
Interesting thing about this one, it can not only create a BIM model on your mobile device, while you are walking around on site; and read data from a handheld measuring device, BUT it can export that BIM model straight into your ArchiCAD software once back at your computer (I nearly said once back at the office, but I suppose your full function laptop could be waiting for you on the front seat of your car in the parking lot on site, getting old clearly).
The software has an extensive range of tools, allowing creation of several building components and intelligent edit and management functions. We have not had a chance to test it fully, but early exploration is looking good.