The harsh reality is that the vast majority of released pets will die. Some will die quickly, possibly after getting hit by a vehicle or running into a predator. Others will die a long, slow death by starvation. Either way, it is extremely cruel to return a pet into the wild. Captive bred animals will not have learned the skills needed to survive in the wild. Even those that were originally wild-caught still have a difficult time adapting to a new environment, where their usual food is not available or the conditions may not be suitable. Life in the wild is no picnic, especially for animals that are used to free food and shelter, especially if they are released into an environment that is not suitable.
Some animals become the exceptions to the above scenario, but a happy ending is still not the usual result.
Ecosystem Damage: Invasive Species
Given the right animals in the right environment, released exotic pets do sometimes find success and colonize in the wild. However, this is often an ecological or agricultural disaster. There are numerous examples of "Invasive Species" -- cases where an introduced plant or animal has become established to the extent where it "takes over" an ecosystem, diminishing native populations. Invasive species can cause problems by preying on native species, competing for limited resources, or by introducing parasites and diseases not normally found in the area. Only a few of the most dramatic cases of invasive species can be attributed to the pet trade, but there are a few locations where released animals have become established and caused damage. Red eared sliders are quite adaptable at colonizing lakes or ponds, and they often thrive at the expense of other species. Parakeets have successfully colonized some areas to the detriment of native species and agriculture. Giant African Land Snails are considered a huge risk of becoming an invasive species due to their voracious appetites and amazing reproductive rates. Warmer climates are often more hospitable to invaders. For example, iguanas and Burmese pythons appear have established populations in Florida. Iguanas are doing a fair amount of damage to the local vegetation as well as becoming a nuisance, and Burmese pythons are feeding on local wildlife and becoming a major concern in the Everglades. Another example is the situation with chameleons in Hawaii (PDF format).
We've all heard stories of snakes lurking in the plumbing or alligators in the sewers. While many of these are urban legends, the scenarios are not that far-fetched. It is not uncommon for snakes returned to the wild to survive for at least a while, probably because they tend to find a quiet place to hide and can go a long time between meals. Every so often news stories crop up of constrictors being spotted in unlikely neighborhoods (and often suspected in the disappearances of neighborhood pets). Some snakes are large enough to pose a risk to people, too. Other exotic animals such as non-domestic cats are also troublesome since they could pose a risk to other pets in the neighborhood, and even people, as well.
Regardless of whether these exotic pets returned to the wild are a danger, a nuisance, or an ecosystem threat, returning your pet to the wild is terribly irresponsible. Never return your exotic pets to the wild!