Want to know how search engines process URLs and Domain names?. Here is a video from Nathan Buggia from the Live Search Webmaster Center.
The Robots.txt file on your website tells search engines how you want them to crawl. Specially which directory or files you don't wan't appearing in the SERPs.
For DNN we should pay special attention to this because you will often see DNN's supporting directories show up in search results.
The easiest way to avoid this by taking this sample Robots.txt file that DotNetNuke.com provides and build your way up from there.
You could use Google's Robots.txt tool to check your Robots.txt file to make sure that all your Ducks are in a row.
Before you go ahead and submit your website to the major search search engines make sure you have all (or most) of your SEO tactics in place. Why? because there is no second first impression. After google indexes your page for the first time it will evaluate when to do it again, if it finds that your site is not "worthy" then it might take weeks or months for it to go and crawil it again.
So make sure you have your Page Titles, Meta Tags, Search Friendly URLs and other SEO Tactics in place before you submit.