There's a popular view that says you should work out in the morning to maximize the impact on your metabolism throughout the rest of the day. It seems to make sense. I can't seem to do it on a regular basis so instead of feeling bad about not following the popular theory I still go to the gym in the evenings and work out like a demon when I'm there. I agree with Nimbletoe in that I wish I could do it first thing in the morning. The few times I've been able to do it it does have a good and uplifting impact through the day.
All that said do what works for you and what you feel you can do consistently. What's best for you may not be the popular view but if you stick with it that's what's important.
One more point on Nimbletoe's post ... I also struggle with sleep if I work out hard past 9pm.
Good luck!
P.S. Hey PLAN9 -- where do you stand on this one?