WordPress.org

Sort Query by Post In is a very light weight (less than 10 lines of code) plug-in intended for developers executing custom post queries. You're welcome to include it in your theme and redistribute - just offer us some credit, please!

When constructing a WordPress post query in your theme template files or plug-in, WordPress offers the option to explicitly specify the posts to retrieve using the post__in parameter. Unfortunately, the orderby parameter does not offer an option that will sort the result by the exact order passed in the post__in parameter. This plug-in adds a post__in option for the orderby parameter that will order the result by the exact order specified in the post__in parameter.

And don't worry about the plug-in being deactivated - your post queries with the new post__in value used for orderby will simply gracefully fall back to the default date sorting.

Authors: jakemgold, thinkoomph