This is a patch to Elgg Core, which will change simplecache, so that it uses a subdirectory in the data directory, with the same name as HTTP_HOST, to store cache data, instead of using the data directory itself. 

By applying this patch, simplecache can be turned on for Elgg installations which change depending on the virtual host name.

This patch has been running on a production server now for several months, with no problems.