Is it detrimental to do cardio on the same day as lifting? I've heard people say that optimally they should be spaced out by at least 8 hours apart, and some people have even told me to do it on completely different days. What do you guys think?

prefferably different days. If you absolutley cant do that, as far apart as possible, doing one in the am and the other in the pm.
If you have to do it in the same session, i would do weights first, and cardio after, though, you really should just do them on different days or the am/pm version.

Unless you are an advanced lifter it really is not going to matter. You will have results if you lift then do cardio right away. For most people it is a timing issue. Going to a gym 2 X per day or every day in order to space things just so does not fit into everyones schedule.

For simplicity, get in, get it done, get out. Results will be there if your workouts are sound.

I think your goals also factor in greatly. If building muscle is your goal, lifting weights and then getting out and getting in proper nutrition is vital, ASAP. If fat loss is your goal, I think a 20 minute run after lifting is fine. I've been doing that for over 2 months and haven't lost a shred of muscle, while losing quite a bit of fat. Either way, I agree with goergen that if you have solid lifting routines, with or without cardio, you will see results...