Dual-band operation makes the most sense if you're mixing video and data traffic on the same network.
With the release of Metalink's dual-band reference design for residential gateways (beefed up with an IXP4xx Intel network processor), I think it's safe to say that the 802.11n era has truly dawned.
There have been other "n" product announcements. But the fact that Metalink has stepped up with an "industrial-grade," dual-band design that is robust enough to handle multiple HD multimedia streams on the 5GHz band and the "legacy" PC data traffic on the 2.4GHz band seems significant to me.
I also think dual-band operation is the way to go if you are going to mix multimedia data with PC data. I suppose you could squeeze a video stream into the 2.4GHz band along with some data traffic but I think you would be asking for trouble.
More information on Metalink's reference design is available at Metalink releases dual-band 802.11n reference design.