INFO ON MOB TOWERS
There is a very likely chance that current mob towers are going to fail miserably because of a change that was released in the weekly snapsnot 12w04a, which more than likely will be released in 1.2. The link is located here: http://www.minecraftwiki.net/wiki/Upcoming_features
Here is the line on the page that disturbs me, it is highlighted with a red box:
I recommend if your mob tower uses signs, fence gates, or paintings to make mobs fall, you might want to reconsider it and switch to one that works automatically pistons.