I believe Telstra in Australia has such an agreement, you can stick a Postpaid Telstra SIM with global roaming enabled in your Iridium phone and use the satellite network, at very significant cost of course, but for very casual use it'd be a lot cheaper than keeping Iridium minutes "alive" with an Iridium SIM.
(regards Telstra, that's what I've read on the web in threads from a couple years back, but may well no longer be possible)
update: Apparently Telstra still does it... http://www.youtube.com/watch?v=eC5wkXORcjs but are any NZ providers also up with the play?
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Comment by freitasm, on 23-Mar-2010 21:36
You'd probably get more replies in a forum thread.
Comment by wellygary, on 24-Mar-2010 08:58
I suspect that the reason Telstra SIMs work is because of the Australian Government's satellite phone subsidy scheme for bush and outback users.