Earnings estimates for companies change just about all of the time, at least for large, widely held companies. It's often hard to get a good sense of how these estimates are changing. One good source to track these changes on a daily basis is on NASDAQ.com. You can see what estimates have changed for the current quarter, the next quarter, current fiscal year, and next fiscal year.
If you need to get a sense of how the direction of estimates has been changing in recent quarters, I know that a lot of brokerage sites have analysis tools that help you do that. Alternatively, there are sites like Yahoo! Finance that show very much the same thing.
Now, what is the usefulness of analyst estimates? That's an entirely different matter.