There is currently a closed beta running. Before that was a dev preview. The participants have been invited. Next step will be a beta with more participants. Details are in the EE Blog.
My advice would be to start by making plugins for the 1.6.x branch. Conversion to 2.0 should be fairly easy later.
Starting to build something for EE in CodeIgniter without knowing 2.0’s API does not make much sense, IMHO.
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