I think it can be a good thing when an artist's song is used to promote something - provided that what they are promoting is something they believe in personally. There's nothing wrong with VIA Rail. Now that they have those great rates along the corridor, I've taken the train several times back and forth to Ottawa in the past year. It's cheaper than the bus, and way more comfortable.
Especially in Canada, most musicians don't make a lot of money, so in my opinion, if they can sell their music to promote something, what's wrong with that? It gives them more exposure. As long as the ads are tasteful, and the music is fitting...
So good for Greg and Blue Rodeo, having such a great song chosen for a commercial. I know, they hardly need more exposure in Canada, and they are already a commercial success, but hey, if it helps them retire before they are 95, helps Jim put his kids through college, what's wrong with that? I wish that same success on other Canadian artists such as the Sadies or Luke Doucet or Justin Rutledge or... well, you get the idea.
But I do get where Schwa_guy is coming from. An artist who sells his music to promote something so that a private enterprise can get rich could very well be construed as "selling out".
On the other hand, though, some songwriters sell their music to ad agencies for a living. One of my friends, who used to have a successful band, does just that now to supplement his income. He got sick of performing in front of people who treated him like a television set (ie, wouldn't shut up while he was singing). The only difference is that the original concept behind these artists' jingles is specifically geared towards what the ad agency needs, as opposed to a songwriter writing a song for his own purposes and having an ad agency pick it up to promote something related to the song.