There's just so many variables to that question. I will ride if there's a 30% chance of rain, but it's not raining in the morning (if I get wet coming home I can just change clothes when I get home).

I also usually attempt to wear something synthetic that will keep me warm even if it's wet.

On a related note, it also depends on temperature - if it's hot out and it rains it's actually pretty fun. Right around 60 degrees is about the worst...I'm more likely not to ride if it might rain then because it's cold enough to be uncomfortable if I get soaked, but not cold enough to comfortably wear my rain gear. When it's like 40 degrees I'll ride because my cold weather rain gear *is* waterproof-breathable stuff (and I spent the cash on enough gear to cover me head-to-toe). If I put more effort into specific clothing I might be able to ride around the 60 degree mark as well with wool or synthetics, at the moment it's just to much hassle.

I would add that one thing people do if their commute is longer than they're in shape for is to find a place part-way there to park their car, then drive their bike partway there, hop on the bike and bike the rest of the way. After a while they're in good enough shape to just bike the whole route. The advantage is that they don't have to find spare time to get into shape to do the route.