In-Series 2.0 is now available!
This version allows for negative numbers when ordering your series, so you can now easily insert items at the beginning of your series without re-numbering all the existing entries. Note that due to an unfortunate bug in WordPress 1.5.2, you will be unable to use a zero.
In-Series 2.0 also fixes a few minor problems with the all_series() template tag.
Thanks to Drew for his assistance debugging this!