How does Twitter select Trending Topics?

This morning around 6 AM, PST, I noticed that the newly announced Samsung Galaxy tablet was at the top of Twitter's world wide Trending Topics list.  It was also at the top of the United States list and number two in the UK.

But, it was not even listed in the top 10 for Los Angeles.  I figured that was because it was early in the morning, so I checked New York and London.  Nothing.

It turns out that the Samsung tablet was not listed in the top 10 of any of the cities listed by Twitter:  Atlanta, Baltimore, Boston, Chicago, Dallas-Ft. Worth, Houston, London, Los Angeles, New York City, Philadelphia, San Antonio, San Francisco, Seattle, São Paulo and Washington, D.C.

Half an hour later, the Galaxy had replaced Stephen Hawking as number one in the UK, but it was still not in the top 10 for London.

Twitter says they will sell the 11th spot on the Trending Topics list to a term that is in the top 1-200.  Do they also sell spots on the top 10?  If they are selling Trending Topic spots without identifying them as paid, will their credibility be undermined?