This is more about the US economy than the Canadian one, I think.
The US debt grows and grows. And since the debt is owned mostly by foreign interests (obviously, since the UC can't borrow from itself and the national bankings system can't absorb a few trillions worth of debt*), that means the amount of USD available on the monetary market keep growing.
Too much offer drives the prices down. I haven't bothered to check in a while but I'm guessing if you look closely at the numbers you'll find that that US$ is losing value against most major currencies, including CAD$, whereas the CAN$ isn't probably gaining that much ground against other currencies than the US$, like euro for example.
Anyway, great news. I've been bugging my comic book shop about that for a while. The previous gap was especially galling since the purchasing power of the canadian dollar within Canada has always been on par with the US$ for years. It's just the exchange rate that was skewed. Which incidently the exporting firms in Canada loved. They have been experiencing trouble of late which shows that a strong canadian dollar isn't to everyone advantage. But me I'm happy.
*For the record, the big bad chinese government is the main holder of US treasury bill. Were they to miss a new emission in these days of deficit and the american economy would take a dive. Just in case you were wondering why Bush isn't nearly as tough with China as he is with Iran. It ain't just the nuclear weapons.