Let's have a look at the web site SmartScoreboard  from my last post . If you click there on Tell a friend in the upper right corner a new dialog will appear where can enter your name and the mail address of your best friend. Before you try this move the dialog to the right border of the window. Now, don't enter any data and click on Send. If you have Fiddler open you will see that there is a server roundtrip done with AJAX (about 52,000 bytes) and when the result is downloaded the dialog box will appear at the initial position (on the upper left corner).
The problem here is that the position of the dialog is not available in the viewstate information which can be used on the server-side code to put the dialog box to the correct position. Maybe here it is easier to use an seperate UpdatePanel only for the display in the dialog, not for the whole page.
Another thing that I have noticed is that the emblems of the teams disappear when clicking on Tell a friend and never come back. If you run the same test on the Help  page it will be different (i.e. it will display the last error message after you clicked on cancel).