OnLive CEO Steve Perlman told Joystiq that this offer, the "first of many," is just a tentative step by OnLive. "You can think of this as cloud game meets physical media 1.0," he said. "This first version doesn't have cross-pollination between your local PC and the cloud."
The main issue preventing this from being done right now -- preventing, say, OnLive cloud saving for retail games -- might surprise you: mods. Perlman said that since OnLive uses slightly different versions of the game than the retail PC, if any kind of alteration is done to the base game, it could cause compatibility issues between the versions. Cloud saving on OnLive "can be done," he said. "It requires more thought than printing a code." He affirmed that OnLive will do that, but "that'll probably be like 2.0."
OnLive is planning not only to solve that issue, but to allow for mods in the streaming versions of games. "I think if everyone wants to do multiplayer where everyone has auto-aim, that's fine, but you don't want to mix games, you know?" He describes setting up rooms in which people can choose to play with mods.
"This is all work that was done many months ago," Perlman noted about the pack-in code, slyly contrasting the lead time required for a retail product with OnLive's instant-on nature. He then laughed. "Oh, my god!"