Well no. Plastic biogrades at a snail's pace. Timescales differ on type of plastic. Plastic bags apparently take 20 years to biodegrade, for instance. PVC pipes are used in plumbing because biodegradation is extremely slow - not that it doesn't ever happen.
I think plastics are one of those things where, dumped into a random place, they probably won't be biodegraded. However, for any given plastic, there is probably some organism in existence which will eat it under some or all conditions. So the usage of "biodegradable" maybe depends on which viewpoint you are interested in.
For example, here's the wiki on polyethylene, which seems like the typical plastic for "plastic bags" from a store:
