I just got the N900 and can't believe this function is missing. For me it is work related. I can set various ringtones for mates etc. And ignore them when at work, but if the missus rings me I know it is slightly more serious or important and can answer just by ring tone. Vice versa, at weekends if the boss calls I can ignore it (unless 'on call') A glowing error not to have it.

I come from HTC phones that have had this for years as standard and It should be added.