This post explains however how Google looks at this practice and what is the best way to have multiple domains coexisting out there that point to the same website. The truth is you should only have one primary / preferred domain that resolves directly to the website and that this should be getting a 200 response which basically means "The request has been accepted for processing, but the processing has not been completed.". http://en.wikipedia.org/wiki/List_of_HTTP_status_codes All of the other domain names should only point to the primary domain using a 303 redirect.(standard URL redirect).
Here is a link letting you know how to post 303 redirects for each of the major platforms;
The reason for this being the acceptable practice is that should all of the domains finally resolve to the same website files/site then you are technically visiting the same website for each URL and this is labelled as 'external duplication' rather than the sticky 'duplicate content' term that we all hear of when first implementing domains for the same website. The 'duplicate content' rule is there for one reason, to prevent people from duplicating their website's and reproducing all of the internet. It is acceptable though to have multiple domains pointed to the same website should you let the search engine indexing bots, be aware of this fact, or if you are not attempting any sort of trickery when it comes to multiple domain use.
There is something I am yet to mention and this is the use of what is called the link 'canonical' property now supported by all of the major search engines. It is basically a format which allows you to publicly specify your preferred version of a URL. It is for use if you are using website URL's to point your users to the same or slightly different pages via the same URL's and will show the search engine bots that you are not trying to trick them into believing that there are two separate website's at the different URL's and works as a notification method almost that there are multiple URL's to be indexed for the same location. It is highly recommended when you have multiple domains pointed to the same website. Here is a great blog post from Google that covers this topic and the syntax.