I've seen the "IPv4 will get more and more expensive, and more and more sites will provide IPv6 connectivity, and (for some unspecified reason) clients will put out the effort to get IPv6 connectivity for no near-term benefit, and then when nearly everyone has IPv6 connectivity, IPv4 support will become like IE4 support and sites will start dropping it, and the IPv4-only people will then demand an upgrade" argument, and I think it has flaws.
* Why do clients - home ADSL users, small offices, wifi hotspots - want to bother with IPv6? It offers them no benefit for at least several years. Everything that's good is IPv4 only, or maybe IPv4 and IPv6. All they need is one external IPv4 - or they can share an upstream IPv4 via carrier-grade NAT, so they needn't bear any "rising cost" of IPv4.
* Why should important sites really bother about IPv6? They already have large IPv4 allocations, and there's endless tricks to make better use of them (they can vhost any HTTP-based service, for a start). Moving to IPv6 will make it easier for startups to get IPs to compete with them. I'm not sure why some of them have offered limited IPv6 access (Facebook's is just a proxy that forwards on the connections via IPv4, it seems), but they don't seem to maintain them well (bit.ly was inaccessible via IPv6 all day; nobody seemed to notice) - I suspect they're mainly "20% time" projects.
* Just how near is the point where it's a good idea to drop IPv4, for clients or servers? There's a lot of legacy networks to shift... more so on the client end than on the server end, which is dominated by a "top 100 sites" or so that could all conceivably add IPv6 support with little effort.