I do invoicing from my van. I dont necessarily use a laptop, though I could, but using quickbooks on two computers for one company is a pain in the butt.
I use an app on my iphone called Invoice2Go. It is VERY easy to use, allows for a paypal button on your invoices, and puts together a very nice looking invoice in .pdf format that you can send via email, all from your phone. All you need for it is 3G service.
On the other hand, I could very well switch my quickbooks over to just the laptop, in which case I would use my iPhone to create a wifi hotspot (for about $20 you can add this feature with AT&T, or, like I have done, you can do it for free if you jailbreak your phone. Afterall, I pay for "unlimited internet".... but i digress) and connect to the internet that way on the laptop.