Don't worry, Dave, yours wasn't the loudest. ;) 0dB is a mine field, because it all depends on which scale you're referring to. If you're referring to Full Scale (like I did in the post), 0dB is the absolute maximum volume you can produce in a digital system before clipping occurs. Someone suggested that in addition to -10dB full scale, we should have 0db on a VU meter equal -20dB full scale. You see where this is going yet?
There are all kinds of measurement systems, and it all depends on how your meter is calibrated. For a while now, the idea of dialnorm (where the dialogue should sit in the mix) has been ruling audio for film and television measurements. For a long time, the US used a scale known as Linear EQ A, you'll see it abbreviated in a lot of places as LeqA. Using that scale, many broadcasters designated a dialnorm of -27dB LeqA, and you need special equipment to be able to get that measurement. A common dialnorm for film was -31dB LeqA.
That measurement system is going away now, as the new suggested spec is BS.1770 (something that's been gaining ground in Europe for a number of years now), which you'll see referred to as LKFS. A very common measurement for television using an application of this system (i.e. ATSC/A85 or EBU-R128) is -24dB LKFS/LUFS. They've switched over to this system because of arguments regarding the way each scale works.
LeqA really only accounted for dialogue volume over time. The program could get all kinds of loud without going over the -10dbfs (full scale) peak limit spec, as long as all dialogue measured out to -27dB LeqA. The LKFS scale takes into account the rest of the audio program and is an indicator of the program's overall loudness level over time.
This is a lot of technical jargon that doesn't really answer your basic question of where to mix your levels to. It's kind of hard to tell you "x value" dB full scale is where you want to be. A good starting place is to keep all peaks below -10dB full scale. From there, it's a question of how much dynamic range do you want in your piece. You should really start by calibrating your monitors. A quick and dirty way is to play some pink noise through your monitors at -20dbFS and use a dB-SPL meter (you can get these cheap at Radioshack) and adjust the monitors until that meter reads [78dB for TV style mixing/83dB for film style mixing] from your listening position. This means that the max your system will output Full Scale is 98dB-SPL or 103dB-SPL respectively (getting close to the threshold of damage/pain with film mixing, but staying below it). After you've done that play a -20dbFS 1kHz sine wave out through the monitors to give yourself a reference point. Then you can load in something professionally (DVD's can be great for this) done and A/B the program vs. the sine wave, and you'll train your ears for an appropriate level.
This is a better way to start out than to look for "the magic dB" level. If you want to find out how loud an audio file is, I'd suggest loading it into your DAW and using a plug-in that lets you find its RMS value (the gain plug-in in Pro Tools can do this). That will give you yet another reference point. Mixing is all about those reference points and dynamic range. Keep that in mind, and you'll get the hang of what you're looking for in no time.
Edit: I will admit that, when working on television programming, I start out by premixing dialog/narration to a comfortable, but present, level. If I run a semi-QC check on it, it regularly hangs in the -26 to -28 LeqA using Digidesign's Phasescope plug-in (measurement time scale/duration of 3 seconds). I then mix everything else around it.