[Nexus S Q&A, Help & Troubleshooting] Android1049:
This happened to me as well. You have to make sure that your phone has its sdcard folder mounted. Type "adb shell" to get access to the phones command terminal. Then type in "mount /sdcard" I think, or maybe it was "mount sdcard". Either way will probably work so long as you are in the root directory of the phone. You can navigate around in the phone just like a normal linux terminal. "ls" shows list of files and folders in the current directory, "cd foldername" moves into a folder, "cd ../" moves back, "cd /" goes to root directory, etc. Anyhow, once you have mounted the sdcard, you can type exit to leave the shell terminal, and then issue the command "adb pull /sdcard .\sdcard" to get the files. I had a custom rom image in the root of my sdcard so the move took quite a while ~15 minutes. On success of the pull command you should see a bunch of statuses like:

pull: sdcard/data/somefile -> .\sdcard/data/somefile

pull: sdcard/DCIM/somefile -> .\sdcard/DCIM/somefile

.

.

.

XXXX files pulled. 0 files skipped

(Some more transmission stats)

Hope it works out for who ever reads this.

Side note:

On my phone the screen was smashed. I replaced it and it still would not turn on, but it was still running. I had a rooted phone with CM10 rom on it. ADB debugging was one (if its off your pretty much screwed). I started up the phone, and plugged in the usb cable. I tried to use both adb and fastboot from windows terminal. "fastboot devices" would echo back nothing, and "adb devices" would echo back my phone's device number but state it was offline. So to get it online I did the following:

1.) pull batttery.

2.) boot into bootloader (hold power and volume up) the phone shouldn't vibrate if it does you have just normally powered it on. It took me quite a few times to get this to go into the bootloader. To make sure you get in the bootloader type "fastboot devices". If nothing comes up you not there.

3.) once verified you are in the bootloader you need to go into recover mode. Press volume down twice and then hit power (this would normally move the cursor on your phone down to recovery and select it if you could see whats happening on the screen).

4.) wait a bit for it to load into recovery, try typing "adb devices". You should eventually see you device show up as online. At this point you may enter the shell as above, mount the sdcard, exit the shell and pull your files.

Windows instuctions:

Prerequisites are installing adb.exe and fastboot.exe via installing the Android SDK on your computer. They will show up in the platform-tools folder of the install location. Add that folder to your environmental path variable so you can run the ADB and fastboot commands from any folder. Install the ADB driver for your phone (allows you to issue adb commands to your phone when USB is plugged in) by plugging in your phone and going to device manager, find your phone which should show up as a exclamation point, right click update driver, click "browse...", then "let me pick...", click top entry something like "show all", and then let the list populate. On the left look for "samsung" and then select the first one that is something like "samsung adb driver interface". And you should be good to go.

Sorry for the long winded post, I just like to try and explain as much as possible.