There's an even quicker way to search within different search engines, using the tab key. From the Chrome help page:
Search a specific site (Tab to search)
Start typing the web address of the site you want to search in the address bar. If Google Chrome has a record of the site's search engine, it automatically offers you the option to search that site. If you remember the search engine's keyword, you can also type the keyword in the address bar. Press Tab to choose the search engine, type your search term, then press Enter.
You need to have performed a search on the site before in order for this to work. I use it to search Amazon, E-bay, Wikipedia, etc. right from the omnibar! THIS (and the omnibar in general) is one of the best features of Chrome, and many people don't even realize it's there. I could never go back to using separate search and address bars - it just seems incredibly clunky.