By default, PowerPoint uses its 'snap to grid' feature to make sure objects line up neatly. That's great for beginners, but irritating if you want to move items more precisely for a particular effect. It seems obvious that you should be able to turn this behaviour off, but in the allegedly more intuitive Ribbon interface, you could waste a lot of time looking.
A post on the official PowerPoint blog admits as much: "It's never been really easy to find, but it only got harder in Office 2007." More practically, it also details how to temporarily move items independently of the grid, as well as how to switch it off snap-to-grid permanently in both Office 2003 and 2007. Microsoft has promised to make this more obvious in a future release, but at least this helps fix the problem if you encounter it.
Quick Try This: Smooth sailing [The PowerPoint Team Blog]