Update Februrary 2014: The following instructions are valid for RTM version of Visual Studio and QFE1 version of .NET Micro Framework as well.
Some people won't install anything that was just released, or even wait for a service pack release before considering using the product. Others on the other hand literally live on betas or even earlier previews of products, often they are ready to reinstall anytime and paradoxically prefer very clean environment, which is kind of opposite to side-by-side versioning. Luckily I belong to the latter group, and we are again entering the magic season where Windows, Visual Studio, hopefully soon followed by SQL Server and Office, are all available as previews and for free. So I eventually ended up with Visual Studio 2013 Preview and no .NET Micro Framework support...
Since .NET Micro Framework releases support one version of Visual Studio only, there is no way for Microsoft to switch to the new Visual Studio until it is finished. This time, however, is the .NET Micro Framework available as an open source project on CodePlex, so adding support for Visual Studio 2013 might be doable by anyone.
Upgrading from 2012 to 2013 is actually much easier than upgrading from 2010 to 2012. The planned editions are the same ones, and no code changes are necessary. Also the SDK changes between 4.2 and 4.3 were sparse, so doing a diff on these two codebases points us to the steps we need to take — and anything unrelated can be ignored (e.g. managed assemblies, porting kit, tests etc.). These are the files that need to be changed:
If Visual Studio 2013 is the only Visual Studio you have installed, you may want to modify the setenv_base.cmd so that it supports it. Find the two places where VS110COMNTOOLS variable is used and add similar code for VS120COMNTOOLS. Alternatively, just extract this archive (ZIP, 26 kB) over %SPOCLIENT%, i.e. E:\src\client_v4_3 or wherever you downloaded the code to.
The codebase does not allow for compiling the SDK straight out of the box, as documented on CodePlex. Prerequisities are Visual Studio and its SDK, any contemporary version should do. The WiX toolset and other tools are not needed, the required binaries are part of the codebase. Native compiler (other than C++ in Visual Studio) is not needed. Please note the steps below are for compiling the SDK only, you will not be able to compile PK this way, or run the tests.
The installer should enable .NET Micro Framework 4.3 (and older versions) in Visual Studio 2013 only. Although you will see the project templates in the express version as well, it will not work in the express, because only packages signed by Microsoft are allowed to be loaded there. Existing Visual Studio and .NET Micro Framework installations should not be affected. Make sure you uninstall your home made SDK before installing the official Microsoft release for Visual Studio 2013.