We have PHP memcache up and running under IIS6, Joomla 1.5.15 and JoomSEF 3.5.5 (all caches are memcache enabled).

When we try to view a page from the frontend, we get the dreaded "JoomSEF: Cache file is corrupted" message. We cannot find any options to change file locking etc.

Any ideas how this can be overcome?

Hello,
the JoomSEF caching system now fully relies on the caching built-in Joomla. Memcache should be much faster, still, the problem may occur when more webserver process is trying to read a cache file at the moment when another process is writing to it.

So I am afraid that there is nothing you can do with this at the moment. Anyway, we will check on this problem to see what else could be done to prevent this from happening.