[kh99]

In the postbit code there's this:

$usethumb = (($this->template_prefix == 'vbcms_') 
	OR ($this->registry->userinfo['styleid'] == $this->registry->options['mobilestyleid_advanced']
	OR $this->registry->userinfo['styleid'] == $this->registry->options['mobilestyleid_basic']));

so it looks like it could have to do with settings for mobile styles (like a style id for basic and advanced mobile styles)? I don't know anything about that stuff.

Edit: Under Style & Language Settings, makes sure Default Style for Old Mobile Browsers and Default Style for Modern Mobile Browsers are "None".