Samsung Galaxy S i9000 (Android 2.1) comes with Samsing Kies Windows software which is supposed to be used for copying files between your computer and the phone.
Naturally, this Windows software 1) is not very good 2) works on Windows only.
However, you can still mount the device as normal USB mass storage. Just set Settings -> Applications -> Development -> Allow USB debugging checkbox on and replug the USB cable. Then open the notifications menu (pull down the top bar on the home screen). Now there is new option USB Connected. Choose it and then choose Mount.
You should be able to access both the phone internal storage and microSD memory card in Finder / Nautilus.
I created directory /media/mp3 where I copied few music files from my computer. I was able to achieve 5 MB/s transfer rate from my Asus Eee 1005HA PC when writing files to phone internal memory. The phone music player was able to pick up them fine.