Having being tinkering with the bundled makefiles for some time now, I'd suggest we convert the build system to CMake (http://www.cmake.org/).

It's a cross-platform build system that generates build files for the platform's native tools (makefiles, MSVC project files, etc.).
The benefits:

- no need to maintain makefiles for all platforms, just one set of CMake files that will support any platform that CMake is available on.

- simplified build procedure - instead of tinkering with the bundled makefiles, run CMake to generate makefiles ready to build on your platform. Additionally, CMake can automatically detect installations of required libraries with specially written modules. For popular libraries, such modules are available with CMake installation and others can be written.

I've already used CMake for a bunch of projects and it does deliver. I can make the conversion. Any interest?

Is there something simpler than CMake that would work?

cmake-2.8.2.tar.gz - 5.1MB
cmake-2.8.2-Darwin-universal.dmg - 27.2 MB

Adding Yet Another Multi-Megabyte Dependency to build bitcoind seems like a backwards step to me.

What's the most popular build solution for other small-ish, successful open source projects?

Does CMake enable, as easily, cross-compilation using mingw?

Yes, see here: http://www.cmake.org/Wiki/CmakeMingw

Other than that, CMake can generate the following (taken from documentation):
Borland Makefiles
MSYS Makefiles
MinGW Makefiles
NMake Makefiles
Unix Makefiles
Visual Studio 10
Visual Studio 10 Win64
Visual Studio 6
Visual Studio 7
Visual Studio 7 .NET 2003
Visual Studio 8 2005
Visual Studio 8 2005 Win64
Visual Studio 9 2008
Visual Studio 9 2008 Win64
Watcom WMake
CodeBlocks - MinGW Makefiles
CodeBlocks - NMake Makefiles
CodeBlocks - Unix Makefiles
Eclipse CDT4 - MinGW Makefiles
Eclipse CDT4 - NMake Makefiles
Eclipse CDT4 - Unix Makefiles

I have used cmake as well as auto tools.  cmake is great for cross platform support and I develop cross platform code full time (Mac, Linux, MinGW, VC++).