I vote for a mobile web interface.
My office has users with iPhones, Windows Mobile, Palm Pre and Blackberry's.
Having just an iPhone app would not be helpful to my office as a whole. I would much rather have a unified web system that any mobie deice with a decent browser could access.
Otherwise, There will eventually need to be an iPhone App, a Windows Mobile App, a Palm App, an Android App, etc, etc. That would be much more costly then the mobile web interface.