I would love to see some of these estimators come out on the iPhone... maybe as a toolkit?
What about an app that has a feature where you can select the customer you are currently on, press START (it records the time you started), and then press STOP (it records the time you stopped) and then you can e-Mail the invoice directly to the customer from the field, or even set up their cell phone number as an e-Mail address ie: email@example.com
letting the customer know that you have finished their lawn work. You could offer it as a free "mobile updates" service or something like that.
I would love to be able to use my iPhone more out in the field.