Actually, I have found that Airsnort cracks both short and long keys in about the same amount of time (1 million to 10 million packets). The number of packets needed is more dependent on the actual code and the point in the pingflood sequence at which the AP is at any point in time (because certain regions of the WEP key space are more susceptible than others).
I haven't really noticed any extra security from using 'clever' HEX codes rather than the ascii shorthand.
However, Lucent changed its Orinoco firmware in September 2002 to stop its cards transmitting weak packets. I have never managed to crack an Orinoco card running up-to-date software.
Likewise for Cisco - the LM350 series of cards are invisible when running WEP.
But you only need one user on the network to be using a card based on the PRISM or some other chipset (eg, D-Link, Linksys, Zoom) and the whole network can be easily cracked.
Anyway, good work Rob, an excellent explanation...