Clarkesworld Magazine has a fascinating post up detailing the last year of handling their slush pile. Here's the breakdown:

Total Submissions 4269
Days open 320
Avg. Subs Per Day 13.3
# of Authors 2721
Highest # of Subs from a Single Author 25

Those numbers indicate the incredible hurdles emerging writers have to overcome to be published in a pro magazine. 2721 authors submitted 4269 stories in 2009 to Clarkesworld. Wow! Since Clarkesworld only publishes 48 24 stories each year (two per month), that means you have a 1% .5% chance of having your story accepted. Add in that half of the stories Clarkesworld publishes are set aside for "established" authors, and that lowers the odds even more for new writers.

Of course, the quality of your writing can dramatically change the odds. For example, somehow I suspect the author who submitted 25 stories to Clarkesworld in 2009 was not creating professional level stories. Still, new writers should keep these numbers in mind when they consider what it will take to become published.

UPDATE: Thanks to Jay Lake and Robert Laughlin for pointing out my mistaken math (both times). Duh on me. Obviously 2 x 12 is 24, not 48. Guess my brain wasn't working too well this morning. And as Robert pointed out, the actual percentage is .56%, which should be rounded to .6%, but I left it at .5 because I hate rounding.