The website FamilySearch.org has been adding to its online immigration collection at a fairly good clip. Since it is free and thus accessible from home, it is a good place to check during your research.
Some ports of entry are complete (as complete as the National Archives at any rate), but some are still works in progress. Be sure and read about the database before you do your search so you can determine if the search for you ancestor's arrival place and date is appropriate.
The link provided below will take you to a list of the current immigration and naturalization digitized record sets. Check on the right of the list to see when the listed database was last updated.