>You'd have to initially get a count of the rows checked server side. Create a page level variable and increment in the row_databound or loop throught the DataTable and then use that when you're creating the js to set the var's initial value. Use Page.ClientScript or ScriptManager to register the js block.
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>You should be able to set the button server side in the Page_Load after you determine how many rows are checked.
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>You would put the var declaration above the function.
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I'll play with it later today, because yesterday I was unable to make it work completely:

http://forums.asp.net/p/1280253/2444463.aspx#2444463