As a Canadian living in the States, do I need a passport to go to Toronto?

I would guess you need to have a passport just to travel between the border. To prove your identity and such. I’m not sure your citizenship matters.

I think it would be coming back to the US that might be a problem. Or has Canada also become that security conscious?

yes. Your citzenship does not matter.

http://travel.state.gov/travel/cbpmc/cbpmc_2223.html

http://www.ppt.gc.ca/index.aspx
or
http://www.ppt.gc.ca/form/pdfs/pptc040.pdf
if not
you will be turn away or place in care of the US Border Patrol. since security has become very tight at all airports and along the bridges and tunnels of late.

I just spoke to my roommate with your question, as she returned from a weekend trip to Toronto yesterday: Yes, you need your passport.

If you’re driving, as she did, it will take about an hour to cross the border because the US officers question everyone in the car and do some searching. They searched her trunk when she entered Canada and quizzed her and her friend extensively when she returned to New York State.

Going across the Canadian border should not be a problem. coming back into the United States, now requires a passport.

Yes, you need your passport in these days of terrorists under every bush or car seat.

Canada has implemented the same rules. You need a passport! Contact your nearest Canadian Consulate.