I wrote an article in August that details how my app Paperless has done in the app store, with sales data:
http://www.47hats.com/2010/08/paperless-so-far-the-apple-app-store/
Since that time, my sales have varied... with my best sales day being when it was reviewed by Macworld, and other days making very little.
Overall, I haven't done nearly as well as the River Of News developer, but I've recently quit my job to become a full time developer. I really enjoy doing it, and felt it was time to move on from my old job to try something new.
Make no mistake though - it takes an incredible amount of work to develop an app, and do the graphic design, and user interface design - knowing that each of those parts must be really really good for your app to be a success. Then when you have the app "finished" you also add in marketing and customer support. Right now I'm working 12+ hour days (but it doesn't feel like it since I'm having fun with it).
There's also the issue of pricing. Since people don't like to pay much for apps - you really have to sell a LOT of copies to do well... with some developers saying that the only way to be successful in the app store is to be featured by Apple.
Which brings us to the big success stories like that of the developer of the game Trainyard (the #3 paid app at the moment), who recently wrote a similar article to mine and has been featured by Apple:
http://struct.ca/2010/the-story-so-far/