dpreview has just published a review of the Canon EOS 550D / Rebel T2i . The review is very good and it seems that Canon really has produced a great entry level DSLR.
I personally do not recommend this camera for serious nature photographers, at least not as your main body. It would make a good backup body for an existing EOS 7D, though.
If you mostly shoot close-up, the 550D might be enough. For birds and other animals, the 7D is a much better choice due to it’s weather sealing, much faster frame rate and better AF system.
If you just start with nature photography and have a limited budget, the 550D may be a good option. When in doubt, it is better to spend more money on good lenses. For example, it is better for close-up photography to have a 550D and a high quality macro lens than a 7D and a mediocre lens.
dpreview review of the Canon EOS 550D / Rebel T2i