Another vote for Nokia - I currently have a 6310, which is the last of the monochrome screen models. No idea how old it is, I've had it for over 5 years now and it's never missed a beat. Had a Sony Ericsson once but got rid of it after less than a year.
The only problem I have with my phone is that it has accidentally dialled the emergency services from my pocket on a number of occasions (in Australia the number is 000 and the 0 key will register even with the keypad locked). So I'm currently looking at a new phone too, but one with the keypad covered. It will definitely be a Nokia.

