Nicholas Thompson has just committed a new module called Global Redirect.
Essentially, the module checks to see if a given node url has been assigned an alias, and if so, uses a permanent redirect to that alias. Currently if you are using url aliasing - say through the path_auto module - you are left with the old numberic url ("node/1234") and the new aliased url. Thus you risk being punished by Uncle Google for serving duplicate content.
robots
The robots.txt file tells various spidering engines, like those used by search engines, what content to index and what content to leave alone.
You don't strictly need a robots.txt file in your root Drupal directory if you are running a public site. Without one, however, your admin log will start filling up with "robots.txt not found" warnings.
A quick solution is to create an empty robots.txt file. Search engine spiders will find the file, will not encounter any disallow rules, and - hopefully! - go about their business of indexing your website.
Yet there is an even better approach. Why not actually list the directories that you don't want the spider indexing or wasting its time on? Think of the printer friendly pages of, for example, book pages. Duplicate content. Duplicate content bad (just ask Google).
