You can build ImageMagick in 64-bit with VS2012.
Just follow the following steps:
Download ImageMagick-windows.zip from http://www.imagemagick.org/download/windows. This file contains the latest source code.
Open configure.exe in \ImageMagick-6.X.X\VisualMagick\configure. If you want to build a 64 bit version of ImageMagick make sure you check "Build 64-bit distribution" in the "Build options". Click "next" -> "next" -> "finish".
After step 3, an VisualDynamicMT.sln is generated in \ImageMagick-6.X.X\VisualMagick. Execute the file "UpgradeToVS2012.cmd" (requires Powershell) to upgrade your solution to VS2012. You can also just open the file VisualDynamicMT.sln without running "UpgradeToVS2012.cmd".
Edit: With the latest version of ImageMagick this step is no longer required.After opening the solution make sure you change the platform in the dropdown to x64 if you checked "Build 64-bit distribution" in step 2. You are now ready to build ImageMagick.