Making a lava trap isn't that hard -- all you need is a bit of lava, some construction materials, and a couple of ladders. What you want to end up with is a system like this (cross section):
       
   
    
The blue/indigo blocks represent the mob-laden water coming off of your mobfarm collection levels; the stick is a ladder which holds up both lava and water but allows items to drop below, and the red/orange blocks are the lava blocks (they only spread 3 blocks from source). The mobs will be forced into the poking out section of lava and killed, and the items should drop below the lava and flow out through your desired collection canal area.
I'll edit in a picture from Minecraft of the cross-section later; i'm in a bit of a hurry right now.
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What use is an ear that does not hear, or a mouth that does not speak?