First, a user by the name of hounsell on the XDA Developers noticed one of the recent additions to the Windows Phone 7 Marketplace, Network Profile by Samsung, used native code, as opposed to Silverlight-managed code. He then went on to document the application characteristics that gave it its native capabilities, including a DLL called "Microsoft.Phone.InteropServices" that provides COM access.
Building on this, Australian developer Chris Walsh (via istartedsomething) managed to prove it is possible to run native unmanaged code on a retail Windows Phone 7 device. He coded and deployed a WP7 application with native code using the developer sideloading process. The method requires a Windows Phone developer account and registered device to load the application. All that remains is writing a solid jailbreaking application and figuring out a way to distribute it (it's not like Microsoft will let it on the Marketplace).
Walsh is promising tutorials on hooking into the file system and registry access. He notes, however, that apps using these hooks are still running as managed tasks, meaning the kernel can still kill, suspend, and so on to any of them, just like for any typical WP7 app.