I'd appreciate a --ignore-table=tablename option as well, similar to mysqldump's. It can't be an uncommon use case that you want to backup all table data except, say, sessions.
Why not use YUI and jQuery both? That's what I do. YUI's widgets are simply the best out there, and jQuery's selectors are the tops. Combine the two and get jQuery's beautiful syntax with YUI's fantastic widgets. Heck, since YUI is so modular in terms of code files, you don't even need to bother loading all the extra parts of YUI that you'll be using jQuery for.
BTW, big benefit of jQuery over Prototype - jQuery leaves object prototypes alone, making it easier to mix and match other script libraries.