A WebmasterWorld thread has new discussion around using the hash tag, in the URL, instead of dynamic parameters such as questionmarks and the like for special tracking purposes. Typically, tracking URLs for ads and campaigns lead to URLs with questionmarks, ampersands and other dynamic variables, which can be seen as unique URLs to search engines. But when you use a hash or pound sign in the URL, search engines see anything beyond the hash mark as nothing. So there is no real duplicate content issue with using hashes when compared to other dynamic URL variables.
I wanted to share one place I noticed does this. Search Engine Land seems to do this on their logo (and likely other places). Click on the logo and it adds a #selogo to the end of the URL.
This way the URL http://searchengineland.com/#selogo is seen as http://searchengineland.com/ to the search engines. If they tracked those clicks as http://searchengineland.com/?selogo, then that would technically be a unique URL and page to search engines. Of course, you can use parameter handling in Google Webmaster Tools to patch this up, but why bother?
Forum discussion at WebmasterWorld.