@Ronnie: That has nothing to do with enabling it in the GUI — that was something one needed to do to enable it back before there were GUI options. If a build contains IPv6 support, and is recent enough to have the IPv6 GUI pages, you'll see it automatically — it's not hidden.
@eangulus: Basic, preliminary, low-level support was added for certain builds types last September. In that respect, Wikipedia is accurate — IPv6 support was there, in a rudimentary way. The most recent release, Build 54, was back on November 30th — all of the IPv6 GUI options in TomatoUSB were added after that, which is why you haven't seen them in any official releases. Part of the reason for the longer delay since that last release is the desire to get things right before making it "official".
Toastman and Viktek's builds are based on more recent TomatoUSB code which is why show those GUI pages — think of what you see there as a work-in-progress snapshot of it.
When the next release does arrive, IPv6 support will still be limited to only K26 builds and only certain build types — it takes up space, and on devices with only 4MB flash that means tradeoffs must be made.
The above page has a handy chart showing which builds contain what. It will probably be revised before the next release, with a new (or changed) build type that will include IPv6 support but still fit on 4 MB devices.
As for the modem… I assume this is a DSL modem? If so, and your ISP uses PPPoE, then yes, using it in bridge mode should be enough. For other cases, it's harder to say.