Why do people talk about the decline of American power? Where is the evidence?

[Reasonable]

Not saying it's true, but there have been lots of reports charting the balance of power shifting with US power declining. There was a report a few years back from US intelligence agency itself stating this.

To use an apt metaphor its inevitable it will wane a bit currently. US is like Sony with PS2 and China is like MS releasing 360. They only result can be loss of market share (or power) to the new entrant. With the BRIC countries rise in importance this requires a trade off somewhere else and the US is taking the brunt of that.

So with US position globally weakening you get lots of references to that fact everywhere.