While Microsoft is officially neither supporting a user accessible file system nor a registry, we should face it that a Windows Phone 7 is still Windows - even if it runs on a (mobile) phone. And today, TouchXperience - already well known is in still young Windows Phone 7 Homebrew community - has released its first public Beta of Registry Editor for Windows Phone 7. While it's yet not widely known which registry keys can or should be changed for which benefit, Registry Editor for Windows Phone 7 is a full-featured registry editor and works on all WP7 devices. It allows you to explore and search registry entries as well as it allows to create/edit/delete registry keys and values.
Please keep in mind that Registry Editor Beta for Windows Phone 7 is for advanced users only and be careful using it since it provides full access to the registry. So deleting or modifying some values may result in a unstable or unusable system. Nevertheless, since it's not available through the Marketplace but available as a raw XAP-file, either a developer device or a ChevronWP7 unlocked smartphone is required to install it via side-loading.
Cheers ~ Arne