Gameloft is one of the bigger iPhone-specific (or at least mobile-specific) game developers out there, and in fact they're so big, they tell Mobile Entertainment that they're not scared of their development partner Ubisoft and other big game developers headed to the iPhone. Ubisoft developed the most recent Assassin's Creed game for the iPhone, but Gameloft developed the first, and Gameloft's Gonzague de Vallois says that their game had better ratings and sales.
To a certain extent, yes, the App Store creates a level playing field for game developers, and allows companies even smaller than Gameloft to get their games out to a mass audience and gain a following based simply on gameplay, not necessarily marketing or other factors. But despite Gameloft's bragging, some of the most popular titles on the App Store, from Rock Band to Call of Duty: World at War Zombies, are able to tie a successful brand into an app that features solid gameplay. Yes, Gameloft has been able to shine on the App Store. But I wouldn't count big developers out yet -- they're still learning how to use the iPhone's marketplace, and I think we'll continue to see big successes from bigger developer in the future.