I'm pretty sure it's going to be a user error with this one. When you're looking through the viewfinder, you're looking through the lens. If it's sharp when you're viewing it there, then the photo will be.
The issue that you might be having is depth of field. If you're trying to focus on something at f1.4 that's fairly close to you, then you're only going to have about a cm or so of are that's in focus. Any movement by you or the subject will make the photo out of focus.
There's a huge weight difference between the xsi and 7d so you're moving a little more with the 7d than you previously were.
Stand still, take the photo on your exhale, and/or use a tripod if possible.