Matt, et al: I have been getting this since warning message since upgrading from 2.10.0 to 2.11.0-b4 (and it persists through the upgrade to “official” 2.11.0):
WARNING DevicesDetection[2015-02-19 11:26:20] [3d3fb] /path/to/piwik/plugins/DevicesDetection/functions.php(63): Warning - array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer - Piwik 2.11.0 - Please report this message in the Piwik forums: http://forum.piwik.org (please do a search first as it might have been reported already)