To answer your question, no, stock photos are not a viable source of income. You can make money off of it, just like some people who are able to write iPhone applications are able to make huge paydays; but in general the market is saturated, and unless you produce something that stands out amongst all the cruft, you won't be making much.

Think of it in terms of economics.

Supply and Demand.

Professional looking images required (in no particular order):

• Equipment
• Technique/Knowledge
• Skill

And back in the day, all three were fairly hard to come by. Camera equipment was expensive (especially digital), you hard to find someone willing to teach you the ropes, and it took a long time to learn. Now a days:

• Camera equipment costs necessary to produce professional looking images have dramatically decreased since the digital camera boom.
• Likewise, computing power has become more powerful, and much more affordable, and image editing software has become much easier to use
• The internet has made it incredibly easy to find information for learning how to produce quality images, as well as made it super easy to submit images for publication.

As such, the barrier to entry to producing quality images has been lifted nearly completely. With that, more and more people are able to produce nice images. One could say that stock photographs have become a commodity; there is no short supply of quality looking images.

Since the supply is very saturated, that drives the price down (while demand for stock photos has increased, it hasn't been to the scale as supply). Anyone with a digital camera and an internet connection can produce stock photos.

As Nick pointed out that as long as it doesn't cost you anything to submit your photos, you have nothing to lose.

This is outside your question, but if you want to make money off your photography, you're going to want to be creative and see what areas are underserved and target them. Assuming you are in the US, how about getting into real-estate photography--there is a huge glut of houses on the market and (at least when I was looking to buy a house) most have crap photos taken by real-estate agents. Another area, not for money necessarily--is to team up with animal shelters and do photos of the pets for adoption. Most shelters dont spend any time taking nice images, so volunteering your time will

1. Increase your skill in pet photography
2. Let you network and meet potential clients. After all they might want prof. photos done with their new family addition.
3. Help with a great cause: helping find pets a forever place to live.