It works with single installation with CDN turned on and under WP 3.8.1.
However, on my multisite, with CDN turned on, I get a fatal error message:
Fatal error: Class 'QM_Backtrace' not found in
.../wp-content/plugins/wp-smushit/wp-smushit.php on line 102
I get the same error whether in the network or in the sub-site.