Earning Money With AdSense

Hi Steve, I've got a couple questions for you.

Recently, I've heard a few people commenting and complaining about making less money with google adsense in the past month or so.

People on a few forums have talked about a decrease in the amount money that they're making with google, even though their traffic has gone up significantly.
Why do you suppose this is?

Also, what are a few good alternatives to google adsense?

Thanks.