I think the cluster of bricks around power outlets in bank offices is rarely examined nor questioned by anyone with any kind of background. And if the chap sent out to change the printer toner questions it, someone will just tell him someone from IT put it there a few weeks ago...

There have been cases of cleaners putting dongle keyloggers on bank PCs too http://www.theregister.co.uk/2005/04/13/sumitomu_bank/

Miniaturization will only get better, of course.