Well, let's keep things in perspective ..

IMHO, you should not be more afraid of walking in Paris than you should walking in New York, LA or Washington.. (I guess... never went to Washington...).
I'm sure you won't wander at night in some areas, in those towns, without some wariness.

Paris is not (yet ?) as dangerous as Bahgdad (check with Benjamin).
We have yet to bomb Aussie tourists, as in the Philippines (IIRC).
Nor have we got Tchetchenes taking hostages in the Opéra, as in Moscou...

So keep cool.... You won't be roaming the poorer suburbs in the middle of the night, anyway. That could be another story.

Just stay aware of your surroundings and keep your wallet well covered : pickpokets are a really threat, in the Metro (especially when cars are crowded during rush hours) or on the Champs Elysées... (tourists from any country are a god send gift for some people, like everywhere..)

As for civil unrest, just take care not to be caught in between the demonstrators and the police forces... Then I could not predict which ones are the most dangerous in those instances ....

As for passing for a Canadian, it would be OK only if you could speak french, with french-canadian accent..
If not, just keep being an American : not all french people have an anti-american bias. If you keep a normal, sensible human being
profile, you will be surprised, but some of us do really like american people !!
OTOH .... well, now, ...a texas' guy... that could be a problem though...... ;o)) .... just say you are from "Californie" ...

LOL....