I assume this is a reference or source offered at the end of an article. What it means is that the author last checked this reference/source on the given date. This is particularly helpful when listing websites as sources because webpages are often updated and changed. So, when you know the date of access, you know how old the information is.

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Note added at 25 mins (2011-03-15 22:07:40 GMT)
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By the way, here is an example of an article I wrote using the same phrase (see the list of sources at the end):

http://www.onislam.net/english/health-and-science/health/449...

just one more possibility