more or less, the hitbox needs to hit the shield, but not pit himself. So you can't block fox/falco/wolf's side B after they start it. Shuttle loop should be...uh...shield cape-able, but vertical things, while you can flip them if you are careful, may still be able to recover.
Like say you shield-cape toon Link's spin attack. If he isn't moving horizontally to being with(pure vertical) flipping it may not do anything, as brawl allows you to grab ledges behind you if you are close enough.
Actually you
can mirror shield falco/fox/wolf side-B, ive done it before. You can mirror shield the shuttle loop, but its hard and risky and if you mess it up you could end up with unfavorable DI. You can also mirror shield Marth's dolphin slash recovery but again, most marths wont let you do that and its a risky attack to recklessly approach. Most likely this will end up with you hitting the side of the board and falling to your doom before you can do anything about it.
Also noteworthy about the shield:
The shield has a frame of 'super armor' when its pulled out on the ground. Also worth mentioning is that you can tilt the shield to point up ad a slightly higher angle, but not downward. When you pull it out you have until the little shiny glimmer ends to reverse a physical attack and make the opponent face the other way. After that, the attack wont harm you but they will still be facing forward and can just grab or attack again. Also, projectiles will be reversed at a higher speed if they hit during this shiny glimmer. When you have the mirror shield out in the air, you can still move backward or forward as it guards you from direct attacks. I think that if you mirror shield zelda's din fire, it wont hurt you even if it appears that the fire hit your actual person.
Wanna know more about the mirror shield? Check this thread out:
http://www.smashboards.com/showthread.php?t=152410 it will point out anything that i missed or forgot. Use the shield wisely.