Wow, I bet that got your attention!
No, AdSense didn't steal my money, but we are in fact seeing a lot of posts like this in the AdSense forums. And there is a reason for it. Since about April of this year (2010) there have been some display issues with some of the reports pages within your AdSense accounts. Those of you who get cheques every month and don't actually check your stats all that often may not have noticed these issues at all, but they are probably there, in your accounts. In the past month or so a second display issue has cropped up, also relating to earnings, but on a different page.
What we're finding in the forum is that people are looking at the default view, and seeing earnings over the payment threshold (that's $100 US) but aren't getting their cheques or payments...and of course wondering why. In order to see your actual earnings (ie: the correct earnings) on the Payments History Page you'll need to reset the view method from the default 3 months to "all time". To do that, select "all time" from the drop-down box at the top of the Payments History Page and let it load. Now check your earnings for the end of August (or the last month earnings are recorded for). There's a good chance that it is under the $100 threshold - in some cases it might only be a few cents or a few dollars under, but being under at all means you won't get your payment until those earnings are at least $100 or more.
The second more recent issue appears on the Reports Overview Page. That's the page you land on when you enter your AdSense account. If you are noticing the earnings in the "Since Last Payment" option disappearing and re-appearing and fluctuating wildly from hour to hour or day to day, then you are probably being affected by the display issues for this report. And it is a display issue - when your earnings are validated at the end of the month and posted to your Payments History Page, they will be the actual earnings. Unfortunately, at the moment, you really can't hang your hat on the "since last payment" report on the Reports Overview Page.
These issues can be found, with their work-arounds, on the "Known Issues" page for AdSense. Publisher's should bookmark this page, or add it to their Google Reader and check it regularly - particularly if you are seeing anything unusual in your AdSense account. If other publisher's are experiencing the same issues you are, you may find the answer on the Known Issues Page in the AdSense Help Center. Even though the issue might still be inconvenient, you'll probably be relieved to find you are not the only one experiencing it...and that AdSense is aware of it and working on it.
The two display issues mentioned above are being worked on by the techs and engineers. It's probably pretty easy to think the one that began back in April should be fixed by now, but the AdSense systems are fairly complicated and getting the same thing to work the same way for everyone probably isn't as easy as we think it should be. The "Known Issues" page will also tell you when these issues are corrected, so don't forget to check it.