Hey Andrew,
You probably already know about this, and it is not causing any issues, it is just showing up in the logs quite a bit.

PHP Warning: Use of undefined constant is_admin – assumed ‘is_admin’ (this will throw an Error in a future version of PHP) in /home/mysite/public_html/wp-content/plugins/simple-dns-prefetch/simple-dns-prefetch.php on line 20

Thanks for looking at this.
And thank you for a very nice plugin.