Here's my totally untested design for entry in Cathal Garvey's Mousetrap challenge.
(Challenge: http://blog.makerbot.com/2010/03/01/cathal-garveys-mousetrap-design-challenge/ )
(Cathal: http://www.thingiverse.com/cathalgarvey )
Thankfully I don't have the vermin problem Cathal does. The downside is that I have neither tame nor wild mice to test this design on.

The idea is to cut out most of the lid from a peanut butter jar and insert this printed lid. Hopefully it will allow the mouse inside to feast on the delicious delicious peanut butter but not let them out. (Who would want to leave a room lined with peanut butter anyhow?)

For design notes about this thing, check out my blog at www.makerblock.com.

FYI, I think I'm the first one to include a jar of peanut butter as a "part"!

Eat most of a jar of peanut butter
Cut out most of the lid to the peanut butter jar
Print the Peanut Butter Jar Mousetrap Insert
Insert the Peanut Butter Jar Mousetrap Insert with the pokey bits going inside the jar.
Set somewhere a mouse can get to with something on top so it doesn't roll away.
Wait.
Peanut butter covered mice!