for FW 3.1.2, follow evildan's instruction first.

then ssh to your iphone or use mobile terminal and do the following:

$ cd /private/var/mobile/Library/Caches
$ ln -s Maps/MapTiles/ MapTiles

Offline Maps will work again. All you need to do is select Copy (not Link) the map you have. This will copy the map into the cache. Larger map will take longer time to copy.
Then in Maps application, if you don't see the map yet, simply move your screen around with finger or zoom in/out and the offline map will appear.