steve98: If something comes along that replaces the current MySky HDi box then Sky will upgrade existing MySky HDi subscribers who paid the $600 to the new service. Same thing happened when the original MySky was phased out in favour of HDi. Looking at it that way, it's a good investment. I know I'm going to be a Sky subscriber for far longer than the next three years.

Yep, exactly what happened to us. We paid ( well American Express Rewards points did ) for MySky1 for three and half years ago, then July '08 Sky contacted us and asked if we would like MySky HDi - yes please! Delivered and installed the next month for free.
We kept the original MySky1 for multiroom, then late last year decided to do the same thing again ( thanks again Amex ) second HDi delivered and installed, quad LNB updgraded and internally wired to both STB's, all for free.
No ongoing rental, no paying for HD ticket, just normal multiroom charge. We've had Sky for 14 odd years and can't ever seeing not having it, and the two lots of $599 didn't come out of our pockets anyway which is a another bonus.