Jannie,
Its a pleasure meeting you.
I think a pre-paid solution could work, although it would need some capital from iBurst to set up a secure pay site to accept credit card payments. To try and sell vouchers (like scratch cards) and keep it accessible throughout iBurst's physical subscriber areas might be a challenge.
A pre-paid solution would make sense should one travel often outside iburst coverage areas (international) for example when on holiday, where one would ideally want to "suspend" one's service for a few weeks at a time. A really efficient accounts department would be able to suspend and reactivate your account with a few clicks upon request, although I would not try it right now if I was you

). Perhaps consider allowing a m2m subscriber to suspend/activate as many times as he/she wishes, and limit contract subscribers to say a total suspension of 4 weeks for every 12 calendar months. Does this make sense?
Having said all of this, one thing has become clear to most (if not all) of us; your biggest effort should be directed towards optimising (no correct that, start from grass-roots level) your accounts and billing department. You as CEO will undoubtly know that efficient people need efficient support systems to be able to function optimally.
Please view this as positive critisism, but wouldn't offering a pre-paid service just shift the focus from the problematic accounts department instead of fixing it?
I have been a subscriber of iBurst (m2m) basically since its inception, and would personally not be too interested in pre-paid. I am much more looking forward to efficient, trouble-free billing & accounts, faster access times and more bandwidth for the same subscription fees. I had quite a long telephone conversation with Shaun this afternoon about some of my other suggestions, and I am actually quite excited about things to come!
All the best, Rudi