Well, here's an exporter that can export a grid, tree or simply a store to excel. It's a fork from another project, I adapted it to work with ExtJs 4, using the new class system and fixed a couple of bugs.
The Csv exporter isn't implemented and the example and compiled files are not updated (so don't use Exporter-all.js).
It exports all the data in the store loaded at that time. If a grid is used it uses the renderers and column configurations from it.
The download is made through a button and uses datauri, so it doesn't work in older ie versions.
I hope it's useful, if you find bugs or make improvements, just fork it and send me a pull request.