As the Raven says... first morning cardio does cardio/endurance training first thing in the morning (which should, if you are sleeping 8 hours, be a 8-12 hour fast) will enhance lipid oxidation.
Also, while I don't have the study handy, outdoor cardio will burn more calories than inside on the treadmill. Regardless of that, I'd go stir crazy doing endurance training inside except for the pool.
The interesting thing I remember about the study was running at 5mph vs. 6mph amounted to only about an extra 10 calories an hour whereas biking 15mph vs. 17 mphs was like a 100 calorie an hour difference.
I'm almost completely rehabbed now, so approximately 80% of my endurance training is done outside. I was 100% inside for a couple of months during rehab which drove me crazy. I love being outdoors. But sometimes you have to do it inside (like yesterday!)
I don't care about fat loss as much as performance while training so I usually take in 300-400 calories prior to my cardio. When I'm 100% I'm doing 15-20 hours per week training, right now I'm 12-15 hours per week (swim, bike, run, weights). So I typically don't worry about fat burning. I regulate the diet when I get chubby (like now, which really amounts to drinking less beer!)
Just prior to my achilles rupture I started doing trail running a few times a week instead of pounding the pavement and really enjoyed it. So once I'm complete again, I'll most likely do 50/50 road vs. trail running. I'm really interested in doing some ultras on the trails in 2007: Sunmart 50 miler and the Odyssey Off-Road Triathlon which is 2.4 mile swim, 112 mile mountain bike, 26.2 mile trail run. The Odyssey will take almost 20 hours to complete.
Just want to see what my body and mind can stand.