Nothing happens immediately --- the regional authorities still have some free space left, although some are planning to get more stingy. (The discussion here --- http://en.wikipedia.org/wiki/IPv4_address_exhaustion --- is at least a starting point for the curious. FWIW, the first that's likely to really run out is APNIC, for the Asia/Pacific region, within the next three to six months; the RIRs for Europe and North America expect to run out in maybe a year or so, and those for Africa and Latin America expect a couple of years' left of run room.)
Once congestion hits (a few months to a year), I think ISPs natting ipv4 clients, or demanding a premium price from anyone who wants a routable address, is likely to happen somewhat more quickly than ipv6 to the home. But that's based on no inside information...
Also keep in mind there's likely to be a bit of IP reallocation happening. There are a number of companies with full /8 blocks they can't possibly fully utilize.
The Department of Defence Network Information Centre has 151 million addresses. I'm not sure what they're doing with them but they could probably squeeze by with just 100 million if they had to.
Once congestion hits (a few months to a year), I think ISPs natting ipv4 clients, or demanding a premium price from anyone who wants a routable address, is likely to happen somewhat more quickly than ipv6 to the home. But that's based on no inside information...