Wow. Seriously, this looks pretty good, seems it can actually format invoices without having to learn an obscure LISP dialect (unlike GNUcash).
Your site could do with some globalisation though. For example: up here in Canada we have GST as well, but Americans probably have no idea what that is. In Europe/UK GST is called VAT. Your site aside, how well does the application handle the different global tax systems? Also, all your prices for the hosted service are in NZ dollars, it would be great if you could provide a currency select box, or country picker.
I'm a Web designer/developer, so I get grumpy when I see table-based layouts. Since it's accounting software I understand design isn't really your thing (nor should it be), but see how nice your site could look if you switch to a properly designed site, with separated code and layout!
Anyway, not trying to be rude, just offering some constructive criticism. What you've got there looks really promising and I've been looking for a GNU/Linux friendly accounts package. Oh, and give me a shout if you need some help with the design side of things!
Beware of the Turing Tar-pit in which everything is possible but nothing of interest is easy.