If you want this ability for one-off searches, you can use the OR operator.

site:http://news.ycombinator.com OR site:stackoverflow.com <query>

I don't know if there's a max # of sites you can add. I tried it with three and it seemed to work great.

http://www.google.com/search?q=site:searchengineland.com+OR+...

(Yes, I did an ego search. Isn't that the best way to know if the results are likely accurate? :)

If there's a set of sites you want to search through often, you can set up a custom search engine for what you describe. For instance, see this page:

http://www.toddnemet.com/