That's a symbol that represents an unrecognized object (on Safari?). Say if Flash weren't available on your browser, you'd see that.
Poor attempt at a pun.
I see them on the iPhone but actually I've never seen one on iPad. My guess is they just show nothing instead of making it look like something's missing.
There was an early iPad video on Apple's site that showed the lego icon while browsing a website (which they later edited), so it was there at least initially.
Apparently Apple eliminated the icons to make it less obvious that their browsers weren't displaying all of a site's content.