Joel,

Sorry to start with the *typical* answer: It Depends... On whether you want to avoid the post pack.

In order to avoid the post back you'll have to use the onblur and write your own javascript to handle this client side event.

There are a couple of problems with doing this:

1. if a client turns off client side scripting your code is going to fail if the code behin logic depends on the client side script to be functional. You'll have to code a little more in order to catch any exceptions when the client side script fails to execute.

2. Trying to figure out the client side ID has to be determined at the server... therefore, you have to embed the javascript code in your code behind. Not the cleanest solution. I hate doing this but sometimes you have no choice.

The nice thing about coding the javascript is that you keep the user experience smooth and pleasant. A lot more work on the developer.