The Nikon D90 is discontinued but still selling it of. The D7000 is the nearest equivalent and a big upgrade on the D90.
The Canon 600d is a beautiful camera as it's predecessor 550d which is still in production, I think.
The Nikon J1 looks a fine camera but and a big but is the lack of optical viewfinder. The reason I say this is I have the Sony Nex5 which is a lovely camera but in certain light conditions it is hard to see the subject in the screen. Hmm. I believe this is a typical problem with this type of camera.
The Sony Nex are fine cameras but have emount lenses and are not yet made by third parties. And are sooo expensive.
What is certain is you will be driven NUTS with opinions. There are so many good cameras out there and everybody will have their favourite cameras.
I'm saving for the Sony A77 but then I look at the Canon or Nikon dslr equivalents and can't make up my mind.