I already posted this in the GTA III topic in the news forum, but it seems fitting to add it here too.
GTA III is not compatible with iCloud for some unknown reason. Scribblenauts is a perfect example of a game that uses iCloud's Documents and Data function to allow users to pick up where they left off on any iDevice. It's a great user experience to have this feature, especially for a game like GTA
I would suggest anyone that wants this feature to submit a request on Rockstar support
http://support.rockstargames.com/home and also under the app review page.
As apps tend to receive updates based on feedback, if enough people let Rockstar know how stupid they are for omitting this feature I am sure they will come to their senses