Here in Canada, the Canadian Food Inspection Agency is responsible for the monitoring and labelling of all food products in the country. I contacted them about this, and they were quick to inform me that this urban legend is certainly not true. They pointed me towards a list of additives that are acceptable under the Food and Drug Regulations. This list is several pages long, and nicotine is not anywhere to be found. At least the part of the story where Tim Horton's keeps the nicotine levels 'below regulation standards' is clearly not true. NO nicotine is allowed in food products whatsover.
The CBC did a story on the Tim Horton's urban legend, and you can read the official statement from Tim Horton's on the issue, as well as the nicotine test results. Both of these documents are in PDF format.
Another variation of this story, is that Tim Horton's puts MSG in their coffee. This is also false.
Tim Horton's isn't the only establishment that has been targeted for this kind of urban legend. According to Snopes.com, McDonald's has been accused of putting nicotine in their fries and/or hamburgers. My favorite variety of this myth, is that a thin coating of nicotine was put on Pokemon trading cards so kids would become addicted to them.
So what does make Tim Horton's so addictive? Well, I really can't say. It's fresh and it's good, and their drive-thru's make stopping for a coffee quick and and convenient. That's enough to keep me coming back.