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Surely it depends on what you're concealing. An extra navigation section? A huge picture? Probably goes under "Who cares". But substantive content...
If it's present in the page source, the googlebot will see it.
Google supports three different configurations for creating smartphone-optimized websites:
Responsive design: serves the same HTML for one URL and uses CSS media queries to determine how the content is rendered on the client side. This removes the possible glitches of user-agent detection and frees users from redirects. This is Google's recommended configuration.
Dynamic serving: serves different HTML for one URL depending on the user-agent. Use the Vary HTTP header to indicate you're doing this.
Separate mobile site: redirects users to a different URL depending on the user-agent. Use bidirectional link annotations to indicate the relationship between the two URLs for search engines.
Make sure you aren't blocking resources like JavaScrpt and CSS for search engines, regardless of which configuration you're choosing. For how-to on the technical implementation, visit our details page.
Does this script execute before the page in its entirety has loaded
Googlebot (now) ignores it.