Most importantly, Lighthouse 1.2.6 is entirely compatible with Snow Leopard. In fact, the previous version has already been supported by Snow Leopard, but 1.2.6 contains a couple of fixes for Snow Leopard-related issues that could have cropped up in the future.
Additionally, 1.2.6 introduces a bunch of work-arounds for UPnP-routers that do not support the HTTP specification correctly. While previous versions of Lighthouse would report an error with affected routers, 1.2.6 will try a set of different tricks in order to negotiate a way to communicate with the router. This might not always work, but it is a viable way to work with some routers that were previously unsupported.
Finally, a couple of minor bugfixes made it into the release as well, including a nasty crasher related to the German localization.
Link: Lighthouse 1.2.6