What does it mean to be "clipping an r"? Why is that a qualification for celebrities to be invited to the royal wedding?

The Washington Post (April 24) ran an article about the royal wedding under the title, “In London, the royal wedding haters have had enough.”

I was interested in the expression, “the wedding ate the world” which appeared in this article. But I was more puzzled about what “famous person who has ever clipped an ‘r’ is booked for wedding,” implies in the following sentence.

Can somebody teach me what to “clip an ‘r’ means, and why clipping an “r” should be considered a qualification for becoming enough of a celebrity to be invited to royal wedding?

“The onslaught continues. The pundits are only just arriving. --They have already readied their Team of People With British Accents to comment, and comment they shall. Every famous person who has ever clipped an “r” is booked for the wedding. This includes Sharon Osbourne, Hugh Jackman (Australian, close enough), Cat Deeley and Dr. Quinn, Medicine Woman”.

The phrase "clipping an r" here is a reference to the feature of British English known as r-dropping. In many British dialects, including the dialects with the most social prestige, the /r/ sound is dropped when it occurs at the end of a syllable. Dialects which do this are known as non-rhotic, while dialects which retain the /r/ in all positions are rhotic. As usual there's a very good Wikipedia article about it: http://en.wikipedia.org/wiki/Rhotic_and_non-rhotic_accents.

The author of your particular article is noting humorously, that every celebrity who drops his /r/s — meaning every British celebrity — has been invited to the wedding.

To "clip an r" is to say the letter in a clipped style, that is, without finishing the pronunciation. In other words, it's a way of describing a very recognizable element of the "typical British accent". Thus, "every famous person who has ever clipped an 'r'" means everybody of note who you might think could maybe, possibly be British, even people like Hugh Jackman who isn't actually British at all (but people think he is because they can't tell Australian accents apart from British accents).