Official Google Mac Blog: Measuring performance of distributed notifications on the performance of Google Update: "Just how expensive is it? How many notifications can you broadcast per second? As with all Google client products, we want to be good citizens and not bog down the client machine."
A noble sentiment, but dear Google, answer me this: just how many times per second is each app going to be checking for updates? When does this become an important factor, and not a question of premature optimisation? They decided to go for distributed notifications instead of distributed objects, which seems reasonable - not because of the overhead issues (in fact a DO is probably a lot cheaper, if written properly), but because of the kind of information they're trying to get through this IPC.