P.M.
Shane's right about possibly hazardous chemicals being releases from plastic when it's heated.
But most plastic are also absorbent, so using an anti-bacterial soap on them isn't a good idea. The active ingredient, triclosan, can taken up by the plastic and later leach into milk or other liquids. It has also been found to disrupt hormone regulation in animal studies, and contribute to making bacteria more antibiotic resistant. I'd also be careful to avoid any chlorine bleach and scented products for similar reasons; many of the scents used in modern cleaning products are actually toxic synthetics (see more on this at http://www.ewg.org/skindeep/).
You should get excellent results from any of the unscented, biodegradable dish detergents available in most grocery stores (you may have to look in the "health food" aisles. If you want a little extra germ-killing power, you could do a rinse with a bit of hydrogen peroxide, which breaks down into oxygen and water, not residual toxins.