Here is how!

Simply type “all” into the search box on the PubMed home page and hit Enter on your keyboard (or click on the Search button): [type] all [click on Search button]

As of April 13, 2010, there are 19,743,909 citations in PubMed.

Something similar can also be done in CINAHL:

If you turn OFF the "suggest subject terms" checkbox on the search page, and run a search for this: [type] ft y or ft n

The FT limiter stands for "full text", so this essentially searches for all articles with OR without full text available. As of March 10, 2010, this returns 2,628,084 results.