The reason for this behavior is that the when set_enabled() is set to false (as a second parameter) it disables the original button - in such case IE6 will not continue with the postback, which is the expected behavior.
Could you please tell us what is the exact scenario? Changing the text and disabling the button is usually used only when there is Ajax involved - e.g. in such case, you want to prevent the users from clicking the button multiple times. If this is the real scenario, I suggest using the client events of the RadAjaxManager and to disable the button after the Ajax request is initiated. If you want to keep your current setup however, the only way to overcome that is not to disable the button with set_enabled() in your code and to just change the text.
the Telerik team