[TheZenPsycho]

As noted, newer illustrator files are just PDF's (perhaps with extra data for editing).

Older illustrator files are in fact, EPS files. This is somewhat tricky since, an EPS is not actually so much a data format as it is a turing complete programming language. Sooo... yeah, who knows what black magic they did to pull off reliably reading and writing it.

[gcr]

You can read an EPS file by interpreting it and remembering the shapes it renders on the page.

You can write an EPS file by emitting your list of shapes without using the turing-complete features of the language.

Not that hard, at least conceptually.

[TheZenPsycho]

That's all fine and good at least up until you have to kern a line of text. Then what?