A source inside Ancestry.com informs me that they will perform a hardware upgrade Wednesday night, 15-Aug-2007 starting at 11 PM MDT. They expect to be complete before Thursday morning, 16-Aug-2007 at 2 AM MDT. If all goes well, during the 3 hour time frame a user might experience an error on one page followed some time later by an error on another page.
The outage will be instantaneous, so you can immediately continue. Should you experience an error, first try refreshing the page. IE users can press F5 on their keyboard. If necessary as a second step, IE users can press Ctrl-F5 (press and hold down the Ctrl key, press F5, release the Ctrl key). This "super-refresh" gets new copies of parts of the page not normally refreshed.
If refreshing the page doesn't correct the error, redo the operation. Operations might be, for example, adding a person to a tree, searching for a record, or attaching a record to a person in a tree. Clicking links to get to a place where you can redo the operation is probably preferred to clicking the Back button. If that is not possible, click Back until you reach a point where you can start the operation over again.
Contact the Ancestry support website should you experience errors that can't be corrected.