>Oh, your users will hate you. Change the background color of the textbox. Popup a dialog. Do something other than flashing.
>

I tend to agree with you, but I do have one case where flashing is exactly the right thing to do. It's in a very visual application when the user needs to connect one item to another. When she clicks on the first of the two (which is a shape), its border then flashes/blinks until she clicks the second, at which point a line is drawn between the two.

You can see the result on this page: http://www.rflelect.com/products/multi-service-access/ethernet-ip-multiplexer/. Choose the Software tab and then take a look at the bottom left image.

Tamar