The guys from the Game Research Lab at University of Tampere in Finland did research on user-created game content within the modding communities of Half-Life, The Sims and GTAIII/VC (of course).
The preliminary results of the GTA part of the very interesting study can be found in this thread. Comments and additions are welcome.
Link: MC2 @ Game Research Lab
Tags: modding, study, research, community, gta3, gta-vc