I'm not fond of throbbers, but I see the reasoning behind the idea here. Something that may be worth considering is moving the visual, "I'm busy" feedback off the screen. After all, the N900 also has a programmable LED.

A "busy" color & blink pattern using the LED may not be as recognizable to new users as an hourglass or whatnot on screen, but users would likely come to recognize a particular LED light show as "busy" without too much trouble. Also, there would no obscuring of on-screen elements, and as I understand it, there would be no additional load on the CPU once it pushes the proper NJoy string across I2C to the LED controller.