This thread could use a kick start. It is a very important topic annd a great way to develop you skills. You will learn a lot more doing 10 decent sketches than you will beating your head against 1 matte painting.
Here is a recent city. Not very realistic, but it was a fun excercise. I did it in a little over 2 hours with no reference.
And here is a shameless plug for my first dvd on landscape matte painting. This is a sketch that is on the dvd done in about 45 or 50 minutes. I had looked at a few photos a couple hours before doing this, but I had nothing in front of me.