Did anyone commenting here actually read TFA (specifically, the court ruling)?
The reasoning for why the conviction was quashed had absolutely nothing to do with cached images. It was quashed because the police were ruled to have conducted an illegal (as per the Canadian Charter of Rights and Freedoms) search, despite having a search warrant to search for possession of child pornography.
Essentially, this is what happened:
1) Technician shows up to install an Internet connection on accused computer.
2) Technician notices probable child porn links in IE favourites (along with other legal porn links), and sees (legal) porn image, either on browser homepage or desktop. Technician also notices webcam hooked up to VCR (turned off at the time) directed at accused's 3 year old child.
3) Technician returns next day to finish work, and finds computer had been formatted.
4) Technician reports to social worker about possible child abuse. Social worker in turn informs RCMP.
5) Police obtain search warrant based on technician's observations.
6) (Four months after technician's initial visit) Police search accused's home, and find child pornography.
The court essentially ruled that the technician's observations did not legally justify a search. And I find this patently ridiculous.