PeepCode Meet jQuery has been published!
A basics-&-up approach for learning jQuery would be great, with info on overriding other frameworks.
You can use jRails as a drop-in replacement for the prototype helpers but it's not recommended if you prefer Unobtrusive JS. Progressive enhancement is great but sometimes I find it a difficult sell. My spin on it is that is so much easier to debug straight server-side Rails code and then augment that with JS niceties. Of course it depends who you're talking to.
Might be nice to see the LowPro plugin by Dan Webb feature here too. Read a bit about it and it does seem to fill a few gaps in jQuery's armor.
Jeff Whitmire commented
It's easy to remove prototpe/scriptaculous and drop in jquery. After that it's just a matter of learning how to use jquery and knowing what plugins are available. I'd vote for more of a general coverage of jquery, maybe within the scope of a rails project, but there are no rails helpers for jquery like for prototype so for this one you just have to roll up your sleeves and learn the library.
There a couple of very good books on jQuery in general so more value can be accrued from your screen-cast if it focuses on using jQuery within Rails instead of general discussion which the books do a very good job of. Preferably, a non-trivial working rails example should serve as a case study.
Make sure you show us how to override Prototype + Scriptaculous and use jQuery with Rails. I'm learning Rails now, but I'm also super interested in Merb for this exact reason. jQuery rocks.