Google Wave already has its first reported spammer — a marketer for POM Wonderful began flinging flack at all the food bloggers they could find on Wave.
Although at least a couple people are optimistic about Google’s technical capacity to keep spam under control in Wave, it’s ultimately impossible to prevent spamming entirely in any medium without either proactive moderation or closing account creation and forbidding user interaction.
Incidentally, if you want to find me on Wave, ask.