I stumbled across this rather neat service a provider in the UK is (beginning to) offering.  You buy a SIM off them (you can even upload a graphic that they'll print on it for you), put it in any GSM phone (even a $29 dumb one), provide SIP account credentials and their system basically turns that cell phone into a SIP extension.  As far as the phone is concerned it makes and receives normal GSM voice calls but to your SIP IPPBX it looks like a SIP client/extension.  They handle inbound and outbound TXTs with HTTP calls.  You can even change the operator name that shows on the phone (excluding iPhones).  Website for it is http://aaisp.net.uk/telecoms-sip2sim.html.  The geek in me just can't get over how neat this is :-).  As an employee of a company that has quite a few on account mobile phones I could see this being quite useful for the people whose cellphone calls are mainly between the office and themselves, being able to just dial an extension and receive calls with an extension as the caller ID (also significantly cheaper international calls).

I don't suppose there are any innovative, technologically savvy cellcos/MVNOs/Other in New Zealand that could offer should a service.

It's kind of a much bigger version of what OpenBTS and IP-DECT systems do (except for DECT in the case of IP-DECT).

(they can also take your cellphone data and terminate it on a L2TP server of your own choice for routing on to the Internet or otherwise, which is just as neat :-) ).