If you don't need to initiate a post back when you press this button, then making the overhead of a server control isn't necesary.
<input id="addButton" type="button" value="Add" />
<script type="text/javascript" language="javascript">
$(document).ready(function()
{
$('#addButton').click(function()
{
showDialog('#addPerson');
});
});
</script>
If you still need to be able to do a post back, you can conditionally stop the rest of the button actions with a little different code:
<asp:Button ID="buttonAdd" runat="server" Text="Add" />
<script type="text/javascript" language="javascript">
$(document).ready(function()
{
$('#<%= buttonAdd.ClientID %>').click(function(e)
{
showDialog('#addPerson');
if(/*Some Condition Is Not Met*/)
return false;
});
});
</script>