I think you have it slightly wrong, it does not depend on growth but on repayment being relatively assured.
If everyone has collectively lent more than they collectively have and secured them on "assets" that have no value, on what basis other than an assumption of growth can repayment be assured, relatively or otherwise?
Discussions of MBS's, CDO's and other instruments of Magic Wand Finance, and fractional banking models are irrelevant here. It isn't about disagreement over the value of debt instruments. It isn't even about the growing consensus that their value is close to zero. The issue at hand isn't the failure of a particular form of financial instrument or model. The issue is the failure of the structure within which any such instruments and models are possible.
The financial system and the physical energy/matter system are completely separate intellectual systems that are universal and overlapping and which, for the last two centuries, have grown exponentially. This has spawned a number of weird assumptions: that the financial system sustains the matter energy system, that growth can/will continue indefinitely, etc. Most people ignore the physical system entirely, with comic effect (witness the deranged mindset of certain economists who assert that a physical, finite, unsubstitutable resource can be extended by simply demanding more of it).
They are incompatible. The financial system is abstract and can (and must) undergo indefinite exponential growth. The physical energy/matter system is bounded by physical constraints and cannot - the unique properties of hydrocarbon energy sources have allowed the matter/energy system to undergo a few tens of doublings (giving rise to the illusion of correspondence with financial growth), but that time is over now. The current price inflation is an inevitable consequence of the growing disparity between these incompatible systems (the ratio of money to physical outputs).
The CDO/mortgage fiasco is just the mine canary for the resulting large scale financial instability that such inflation generates. As the system trips over in the next few months from non-payment stage into large scale default and repossession it probably will be the trigger, but there are many others.
For example, this month the Federal Reserve gave the corpse of Bear Stearns to JP Morgan. This wasn't altruism, or Federal "intervention" - this was a desperate attempt to prevent Bear Stearns "assets" going to auction and being revealed to be worthless, instantly rendering similar assets by other big banks similarly worthless, triggering a universal margin call that collapses the system. There are an unrescuable number of financial institutions in the same position as Bear Stearns who will, within a few weeks, go public - and this is not a scalable solution.
Tackling the administrative complexities of who currently owes what to whom as the CDO fiasco unravels provides a gratifying sense of "tackling the problem", but the problem isn't being tackled and will shortly undergo a step change in complexity.