To answer your first question, yes, there are introduced, established populations of chameleons in Hawaii (Ch. jacksonii xantholophus & Ch. calyptratus), California (Ch. j. xantholophus) and Florida (Ch. j. xantholophus & Ch. calyptratus).
As for the rest of your questions, the release of any nonnative animal is completely illegal and dangerous to the local ecosystem. There are hundreds of examples where introduced species have been highly damaging to local ecosystems and while there are plenty where no obvious negative impacts have been shown, the risk is simply too great and some degree of competition with native species is inevitable. While I doubt a chameleon would directly cause an extinction or heavily throw off the balance of local ecosystems, it could in theory happen depending on the fragility of the ecosystem and its balance. They are trying to eliminate the introduced veiled chameleon population in Hawaii because of fears they could predate local, endangered bird species which are already suffering from loss of habitat and pressure from other introduced species. Stemming from concern over establishment of introduced species, many states have banned many species to prevent careless keepers from releasing them into local habitats (for example, it is now illegal to sell red-eared sliders in Florida for this reason). There really isn't any real positive to doing this.
Chris
