Why confess now? If you're running a Ponzi scheme, and investors are starting to ask for their money back, why not do what other, putatively legitimate, hedge fund firms have done and announce that you are restricting withdrawals for a few months while you sort out illiquid positions? You'd still have the problem that your actual assets are significantly less than what you have claimed, but, again, why the need to confess the scheme? Let's say that the returns you've claimed imply that you have $50 billion in assets under management, but you only have $15 billion. Wouldn't 2008 be a convenient year to blame the difference on suffering catastrophic investment losses?
I guess Madoff's sense of guilt caught up with him, but it seems like an awful year such as this one would be one in which it might be possible to unwind a Ponzi scheme without getting caught.