[...] if I am authorized to use a wireless network, and after authenticating myself I use something like wireshark or airopeek to sniff packets, how is this any different than just wardriving open wireless networks w/o authentication?
Wardriving generically refers to the activity of identifying accessible access points. The term 'wardriving' is a play on the old 'war-dialing' techniques (amazingly, still worth investigating on a pen test). With war dialing you would systematically dial every extension in a given exchange, or subset, looking for numbers that provide a handshake response (modems). War driving is essentially just driving around with a laptop that has a good wifi card (see: Alpha :) looking for WAPs that are accessible. These activities are external to any single given network. At the point where you access a network you have gone beyond the scope of wardriving.
The other activity that you mention, packet capturing, is fundamentally different in that it is an internal activity and presumes a persistent level of access to the network.
To clarify through example, posit; you're driving down the street and your buddy is in the back seat with a laptop, a wifi nic, and a cantenna. As you drive down the street collecting ESSID information- you are wardriving. This is recon, in a pentest methodology. Once you identify a network to access, assessment, you are no longer war driving. Now you are evaluating a specific AP. Once you have accessed the network... LEGALLY, of course :), and run your packet capture software you are performing internal recon/analysis (AKA 'footprinting').
Yes, capturing a broadcast beacon packet is fundamentally the same as any other packet but the differences I point out are germaine in the context of the original post.
Best of luck.