Just for a little status update on Yesod itself: there has been very little activity on the code side. I've added support for both 32 and 64 bit integers in the persistent SQL backends, but have otherwise been focused on documentation.
I believe we're at the point where the APIs are ready to stabilize. I'm beginning to think that the next Yesod release will be a version 1.0. As such, if anyone has any API changes or feature requests they would like to see, I recommend you let me know as soon as possible. I don't have any timetable on a 1.0 release, but I would like to start getting a basic roadmap down soon.
Right now, I think the two biggest areas to help out with are documentation and testing. If you notice things in the Yesod book that could be clarified, please leave a comment. I'm also planning on adding a wiki section to the site, which should make it easier to contribute. As far as testing: the Hamlet, Persistent and Yesod packages all have their own test suites: if you see something that could be added, send me a patch.
One thing that jumps to mind as a feature for me would be adding more authentication support, in particular OpenID 2 and OAuth (for Twitter). The one thing I'm trying to avoid is adding C-library dependencies, as these can make it much more difficult to get started with Yesod. I believe that currently available implementations of those two protocols rely on a system OpenSSL library, so this will require more thought.
A 1.0 release is important since it marks a level of maturity and stability in the project, but it does not represent any kind of finish line. Once Yesod is considered more stable, it should be easier to build on more support tools. Some examples would be externally available subsites (such as a wiki or blog engine), automatic memcached interaction for caching static content and database queries, and more persistent backends. And I'm sure the best ideas are the ones that I haven't even thought of yet.