You can register up to 6 devices a year (xbox360's, computers, roku boxes, whatever). The only thing to be careful about is that if you do register a computer and decide to wipe your hard drive/re-install windows, that'll use up another one of your registrations (it's be nice if it was like itunes where you could log on and register/de-register computers.
The other thing (that hasn't been mentioned) is that you can have as many simultaneous streams as the DVD allowance on your netflix account. For example, I have the standard 3 DVD out at a time plan. So I can have 3 streams going on at the same time.
Computers are different now. I spoke with a netflix tech a month or two ago about that. Now, computers if they are not used for one week, the device is then put back into the pool and can be used again. Not so for the other devices. Only 4 hardware devices are allowed per year. Computers can be up to 4 as well but like I said, when one isn't used for 1 week, it is put back into the pool to be used again.