I played around with the iPhone 5C today. Played all our HTML5 games on it, everything seems fine except for the top address bar thing. It stays on top and minimizes to about half its width upon scrolling.
If the games are in landscape mode and one loads in landscape, then yes, 1/5 of gaming real estate is lost due to controls.
I'm sure all the devs out there have submitted bug reports on this. In the meantime, let's redesign our landscape games into portrait.
On the other hand, existing portrait games are doing just fine. The address bar isn't taking much space, and if any case, it helps with branding your website. If your domain name is short, like m-game.net, users are going to see it more clearly.
Two thoughts on this:
Privacy - I believe Apple designed the Mobile Safari in a way that forces the user to take into account, the URL of the site she is visiting. It's Privacy 101, where the user is in charge of seeing what domain is serving the content.
Branding Opportunity - Content portals, Carpe Diem! This is your chance to inject branding into your URLs! mhtml5.com, m-game.com, xyz.com - the subtle message will enter into the user's subconscious, and chances are they'll type in your URL the next time they want to play your game!
To sum it up, iOS7 for HTML5 gaming is still going strong. The Apple Devs have some fixing to do, but HTML5 game devs need to manage their expectations and design under constraints. We've been designing games for years under constraint, so what's the big deal?
I'm optimistic about iOS7.