I've recently looked for this as well. Came up with a load of Javascript to be inserted until I found the following:
At the top of your .aspx codefile, insert the following:
MaintainScrollPositionOnPostback="true"
so the very first sentence in your .aspx starts
<%@ Page Language="C#" MaintainScrollPositionOnPostback="true" AutoEventWireup="true" CodeBehind="Default.aspx.cs"
This works just fine for me without having to add any other code for keeping scrollbar positions using updatepanels
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