Original Airport Extreme and Time Capsule models (ones that didn't support simultaneous dual band features) were originally stuck with firmware version 7.4.2 which had a bug that prevented 6in4 (HE style) tunnels from working when the router had a DHCP address (virtually all home setups). This bug was fixed in 7.5.0 and above but never backported to the original hardware.

Apple released firmware update 7.5.2 today for *all* Time Capsule and Airport Extreme hardware that returns usable 6in4 tunnel capabilities to older routers.

Yay!