So far Samsung has been rolling updates out using its windows only software Kies, and not over the air like the rest of the android world.
Is there anyway that non windows users can get these updates?
I'd get a virtual machine and run windows in that for updates. Will probably be the easiest for you, if you're not comfortable with flashing. Usually the ROMs for Samsung come out as Odin packages (program Samsung uses for flashing i guess).
The forum for Galaxy on XDA has alot of resources to get you started. http://forum.xda-developers.com/forumdisplay.php?f=665
If the above doesn't help you, you can use a custom recovery. ClockworkMod Recovery has just been updated today. Follow the steps on http://forum.xda-developers.com/showthread.php?t=734166. With this you will be able to flash updates from Samsung when the community releases them as update.zip, from your sdcard.
This REQUIRES root on your device, but is probably your only solution without Windows. Before you start with this, sit down and read the resources on the forum. Flashing a wrong firmware COULD damage your phone. Be aware of your phone version. The above guide is for the Galaxy S i9000 International version.
Going through the Galaxy S Flash/Root/ADB/ROM guide on XDA, I just spotted this:
Which takes us to here Heimdall - Cross-Platform Open-Source Flashing Software:
Hopefully this will help us all out!
Though obviously as this is effectively a replacement for Samsung's Odin software it's more designed towards people who are happy grabbing their ROMs from various places across the internet and flashing them on in a fairly custom way, as opposed to Kies which is (supposed to be) pretty much plug and play.
If you can find someone or somewhere who has a direct link to the update, you can probably download it to your SD card using your mobile browser. Im not sure what the key combos are for the Galaxy S to get to your Recovery, but im sure with some web searching you can find out how. Once you are in Recovery, you can flash the update. I am basing all this on how it works on other Android phones but have not actually seen it done on a Galaxy S variant. You may try this site for Samsung Firmwares: http://www.samfirmware.webs.com/