Thanks folks. That was helpful.
My question was also about, does an unlocked phone really needs to be rooted in order to gain su access? My impression was it wasn't - since the phone wasn't tied to any carrier.
There are a couple of flavors of "locked" which is what may be confusing you. There is the carrier lock, where the phone is tied to SIMs for a specific carrier. Have an AT&T carrier-locked phone and pop in a T-Mobile SIM and you get nothing. The websites you see advertising "unlocked" phones are advertising phones that have had the carrier lock removed. AFAIK none of the Nexus phones have a carrier lock, you can use a T-Mobile Nexus S on an AT&T network, though of course you don't get 3G access because the frequencies are different. But voice, 2G, and WIFI works fine.
The other flavor of "locked" that you'll see bandied about around here is the bootloader lock. This prevents the bootloader from using unauthorized ROMs. The Nexus 1 and Nexus S both have a bootloader lock, like every other Android phone out there. As shipped, they will only install official ROMs from Google. However, there is a program that ships with the Android SDK that will remove the ROM lock from a Nexus phone. It will void your warranty, however.
Once the bootloader is unlocked (at boot your phone will show a picture of an opened lock) then you can proceed to root your phone. su access *is* rooted, it doesn't really have anything to do with the carrier lock or bootloader lock. su is the program on unix and variants(Linux, BSD, Android) that switches you into the root account. There's a minor complication on Android, which is that many of the programs that say they require root actually need a bit more than that, they often need an auxiliary program called busybox installed as well. Busybox provides utilities for root programs, and can be downloaded for free from the market.
A lot of people aren't clear about the distinction between the unlocked bootloader and root access, and refer to them interchangeably, which adds to the confusion. If they're using something like an HTC or Motorola phone and can't get root access without a fork bomb exploit, then they probably unlocked and rooted their phone at the same time using a one-click rooting program, so they probably don't even realize that there is a difference between the two. But they are definitely two different things, you can have a phone with an unlocked bootloader but not be rooted, and you can have a rooted phone with a locked bootloader.
