I work for a design/build contractor and use my iPad every day. It is not only my calendar and email when I am on the road, but I also use Logmein to access my computer at the office to view old emails, documents and drawings.
I use Pages to type my site meeting minutes, and I both edit and distribute the minutes directly from the iPad. I also use Pages to record deficiencies during final walkthroughs and immediately PDF them to the site super (no more waiting until I get back to the office, typing them up and mailing them out).
Adobe Ideas is perfect for marking up drawings on site. I convert my PDFs to JPEGS and sync them to my iDisk, copy to the iPad's native album and import to Ideas. Sounds complicated, but it only takes a minute and this allows me to quickly and legibly mark up drawings in multiple colours and distribute them in PDF to my trades and supers.
I also use iBooks for reading and searching RFPs on site.