You might look at the following:
http://www.worktrack.com/default.aspx
They provide an application that runs on Palm (maybe others) which allows you to create a Quickbooks integrated invoice, email it, and even receive credit card payment... all before you leave the customer's house (if you believe the sales pitch). I was intrigued enough to see a live demo about a year ago (their office is nearby) and it looked pretty compelling.
However, it required a lot of pre-work in getting Quickbooks set up with all the data needed so that the Palm user could enter the items (material and labor) from a drop-down list. For example, if you did a service call that took 2 hours of labor and you used 10 different types of material (e.g. 1g cut-in box, 14-2, wire nuts, device, plate, breaker, etc.), you would have to have all of those parts and labor pre-loaded into Quickbooks so they were available on the Palm device. Once everything was pre-populated... it really appeared to work just as advertised. Before leaving a service call you could email the invoice, receive credit card payment - and it's already entered in Quickbooks.
The reasons I haven't adopted it are:
1) My Quickbooks does not yet have all the thousands of material items loaded into it, and I'm not sure how the application would handle that.
2) We don't do a ton of service work so it's not really our focus.
3) Palms are pricey and don't want to buy new ones too often if they are fragile.
4) And perhaps the biggest reason - I'm not sure it would be easy and user-friendly enough for the guys in the field to actually use it instead of just writing up a service order.
Would love to hear if anyone is actually using it.