Some facts:
GTA IV MP is limited to 16 players. You have numerous settings and can enable everything in multiple levels, like cops, pedestrians, traffic, daytime, mode etc...
GTA:SA and GTA:VC have community mods for multiplayer. It got really going with SA and they are not as limited as cjp thinks. You can have up to 200 players with SA:MP. From racing to deathmatch over stunting and anything else can be easily integrated via a scripting interface. I played a long time on several Roleplay servers and belonged to the famous Godfather server. The game had been converted into a fullblown MMORPG and/or/including massive battles. There where and are problems with highpingers and hackers, but this is inevitable since this is a community mod. Improvements are made constantly.
Rockstar officialy supported the modding community and said they where OK with it and simply did not have the ressources to bring such great things as multiplayer to the game themselfes.
Quite frankly, as great as the multiplayer may be with GTA IV, I think 16 players is a waste of the city. Once you played with 100+ people and did some decent roleplay or faction based games you will agree.
With the PC release for San Andreas it was the same. They said they didn't have plans for it (I bet it was and is to boost console sales). A few months later they were working on it and around 6 months after the initial release the PC version came out. I don't understand why people think because they say there are no plans for another version that there will never be such a version.
Converting the platform should be A LOT easier than with the PS2 as main platform. Todays consoles APIs are much nearer to PC tech than before and the XBOX360 is basicaly DirectX on steroids.
Everybody gather around me and take 2 cents!