Just change the file extension to pdf.

I've been using this trick for well over two years. Actually ever since the day I wanted to write a PDF parser and saw this upon opening a PDF file with Vim:

%PDF-1.4
%âãÏÓ

This was the first line. Obviously the next thing I did was to rename the AI file to PDF. It was quite obvious that it was indeed just a PDF file since the thumbnail in Nautilus instantly appeared :)

Yes it can be opened by anything, like Gimp, not the layers don't work.

Bonus for Ubuntu users:

rename -v 's/\.ai/\.pdf/' *.ai

Have fun.