Is there a way to limit the people picker control in InfoPath to a certain SharePoint group? I need to do this without writing any custom server code.
Yes, with People picker field in 2010, you can do it in two steps:
Just be aware that when selecting a SharePoint group InfoPath uses the ID of the group, not the name. That means that if you change the group name the picker will still work as expected. If you develop in one environment and deploy to another, it most likely won't.