The official position seems to be that this isn't something you'd ever "want to do". See this thread on the Android Developers list. Google envisage android running on a variety of different devices (CPUs, displays, etc). The best way to enable development is therefore to use (portable) managed code that targets the Dalvik VM. For this reason, the Android SDK doesn't support C/C++.
BUT, take a look at this page:
Android includes a set of C/C++
libraries used by various components
of the Android system. These
capabilities are exposed to developers
through the Android application
framework.
The managed application framework appears to be layered on-top of these libraries. The page goes on to list the C/C++ libs: standard C library, media, 3D, SQL lite, and others.
So all you need is a compiler chain that will compile C/C++ to the appropriate CPU (ARM, in the case of the G1). Some brief instructions on how to do this are here.
What I don't know is where to find descriptions of the APIs that these libraries provide. I'd guess there may be header files buried in the SDK somewhere, but documentation may be sketchy/missing. But I think it can be done!
Hope thats useful. For the record, I haven't written any native android apps - just a few simple managed ones.
Andy