If you want to use this in tandem with SMS PopUp, I'd suggest using ChompSMS instead of the standard Messages app and SMS PopUp, at least on Sprint HTC Hero. Using SMS Unread Count+SMS Popup caused some sort of error where I'd reply to a text on SMS Popup but the badge indicating how many unread texts wouldn't change, and nothing I could do (short of removing the icon and readding it) would reset the count. However, using ChompSMS gives you unread count and pop-up notifications.