Since 1985, the Schengen area has acted as: (according to WIKIPEDIA)
a single state for international travel purposes with border controls for travellers travelling in and out of the area, but with no internal border controls.
If ever anyone had any doubts about the validity of (European) regional integration, this Schengen area pretty much redeemed that notion.
I quite like the preparedness that comes with the application of an EU state as a "Schengen country" (currently at 25 countries now). Wikipedia maintains:
Before fully implementing the Schengen rules, each state needs to have its preparedness assessed in four areas: air borders, visas, police cooperation, and personal data protection. This evaluation process involves a questionnaire and visits of EU experts to selected institutions and workplaces of the country under assessment.
The West African Economic Community(UEMOA)--established 1994--has since 1 October, 2009 made it possible such that "any visa issued by a UEMOA state will be recognised across the Union, allowing the holder to move freely between any of the eight member states, the first stage in the implementation of a common visa by 2011." (http://www.voxafrica.com/en/news/economy/uemoa-moves-towards-common-visa)
At the ECOWAS-level, on 7 May, 2010, a meeting took place in Cotonou, where experts adopted a new road-map for the implementation of the single visa policy within the Economic Community of West African States (ECOWAS).
This will be known as an "Ecovisa", and is touted pretty much like the Schengen visa.
Unlike the UEMOA common visa scheduled to take off in 2011, the "Ecovisa" will take off in 2012. Currently, though, it's possible for West African/ECOWAS citizens like myself to travel freely within the sub-region with simply my passport. I guess a single-entry visa would simply facilitate freedom of movement--especially for non-ECOWAS entities.