Most people concentrate on the price when considering when to stop mining or if it'll be profitable.  IMO, that's only a small part of it.  What few people fail to consider, and is much more important right now, is the difficulty.  Sure, if you're not paying for electricity, you'd always be making a profit... if you don't consider anything else.  When the difficulty goes up so high that you'd be spending a month to get 1BTC with your 5GH/s setup, would you keep on mining, even with free electricity and rigs you've already paid off?  If you still are, you're risking damaging a few hundred/thousand dollars worth of equipment for $100 in profit (I doubt the price of BTC will be that high any time soon).  You'd make more selling that equipment.

If the difficulty keeps on going up at a 33% rate every 10 days, you're probably looking at 4-5 months before you should consider throwing in the towel, unless the price was astronomical.  The difficulty has been going up much more than that and the halving of a block to 25BTC is coming much sooner than expected.

You'd only want to start mining with new rigs now if you believe that the difficulty will slow down or reverse course very soon.