I'm 18.46% mainstream it seems...
Thanks to this, I know can review the mainstreamness of my music taste (as shown via last.fm).
This is constituted largely from a small number of artists well-played on my last.fm scrobbling (which of course does not demonstrate my full range of listening since - unlike the George - my computer is not hooked to my stereo and therefore only shows what I listen to when on the computer and not elsewhere in the house).
Anyway, these are the main culprits for my mainstreamness:
1. Rufus 26.11% mainstreamness
2. Regina Spektor 27.12%
4. Decemberists 43.2%
5. Arcade Fire 58.05%
9. Pulp 21.6%
12. Belle & Sebastian 52.78%
23. Bob Dylan 59.1%
In some respects, this list probably reflects more of the tastes of last.fm users than it does 'genuine' mainstreamness, but there's nothing on this list I'm ashamed of and there's plenty to up my non-mainstream credentials. For me, the rating of 18.46% mainstream seems fair because its less than one-fifth of my playing and I've always liked to mix up the more obvious with the obscure. of course, I suspect that George will be off-the-scale for obscurity [though those pesky last.fm obscurity loving dudes may yet affect him], and apart from Dylan, I doubt that Cloud will be making much impact on the mainstream. But I'm happy where I am.
Update: hmm - seems with George I forgot to take account of the impact of a life-long love for REM... and the impact of The White Stripes (perhaps further evidence of the last.fm listener profile)
Update: whilst Cloud comes out with the top of us three for obscurity with a rating of just 15.69% mainstreamness...