I was feeling a little lazy last fall and decided to directly re-use a week old yeast cake left over from a secondary. There was about 1/4" of beer covering the cake and it had been sitting at about 60F in my basement, with an airlock, kept sanitary since siphoning & bottling the previous batch. I had planned on washing & harvesting, but laziness won the day.
The following week I dumped the cooled wort from the next batch directly onto the cake and fermentation took off within hours. Since it was a secondary cake, there was apparently no trub on the bottom and it looked like 1" of pure yeast.
The cake was a Wyeast 1214 Belgian Abbey Ale. The 2nd batch was great and worked so well that I tried it again with a Pacman secondary cake, but that batch is still conditioning so I don't have any feedback yet.