I think that lots of people trust TIOBE's index, although it has its flaws and disadvantages.

On the basis of what? Do you? :-)

Personally, I have my doubts. Here's some food for thought:

• Looking at the first few pages of Google listing for TIOBE I note that virtually all listings are either to (a) tiobe.com (b) simple statements of fact about the latest results - often by vested interests, i.e. language blogs tooting their language's successful position in the rankings - or (c) significant critiques of the listing. In fact, the first non-tiobe.com Google listing (at #3) is one such critique.
• More telling, after scanning several pages of Google listings I failed to find even one article showing that someone was actually making decisions based on the TIOBE index. Compare that to metrics like COCOMO.
• I sometimes like to scan my alumni print journal database to get a thumb-in-the-air sense of someone's off-line brand recognition. According to this database (a repository of 1300+ trade journals and news magazines targeted at managers), TIOBE has only four articles: three in Dr. Dobbs (2 from 2008, 1 from 2005) and one in ComputerWeek (2008). The content reflects the same trend found in the google hits: a self-promotional interview, a critical letter to the editor, and simple reporting of results. By contrast, the Gartner Group has 1407 articles, many of which use the Gartner Group results as a building block in their own independent analysis.

Best, beth