I make use of multitasking while on the phone relatively often, to look something up; even more often now that I have a Bluetooth earpiece. Recent example, when calling my wireless carrier: "Do you have a land-line I can call you back on so you can get the IMEI from your phone?" "Actually, I can look that up while on the phone; one moment please." Menu -> Settings -> About product, ... (They could use the IMEI to look up the phone's capabilities to tell me whether it could successfully roam on their network.)

However, I didn't create the "End call" option for myself; I just switch back to the phone app and end the call. The person I created this option for doesn't really multitask, though they do leave programs running sometimes. However, more than once they've somehow ended up with the screen blanked but a call active (still not quite sure how), and in this situation the phone doesn't give any visible indication of an active call, leading to a request for some way to ensure that the phone is off.

Potential hardware designers, take note: hardware "answer" and "end call" buttons considered essential for some users. And considering the N900's screen has a significant gap between the right edge and the bezel...