is the svg format (or whatever inkscapes file format is by default) a universal file format that is operating system independent?

It's an open format so you can open it in any software that supports SVG on any OS. All modern web browsers have basic SVG support.

http://en.wikipedia.org/wiki/Scalable_Vector_Graphics

Yes. THe only thing that I would say is that if you add bitmap images to your project (eg photos), then to use them across platforms, it is better to embed them rather than linking them. Often the path to the file will be different on different platforms eg C:/... on windows and /media/... on linux. Embedding the images means that you don't have to worry about this.

In the early days of our webcomic my collaborator was on XP while I was on Ubuntu (we're both on Ubuntu now). There were never any problems transferring files back and forth EXCEPT:

1) As tomh pointed out, bitmap images. This is easier with 0.48 now that it embeds by default.
2) Fonts. If you need a particular font that isn't available on the target machine, you'll need to convert the text to a path to retain the visual appearance. If you do that, I recommend duplicating it first, and hiding or moving the original somewhere so that you've still got a text version (as well as the path), in case you need to edit it later.