I got this from http://www.baby2see.com: Most of the time in identical twins the zygote will split after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ('mono/di') twins.These twins are very similar genetically, and share a single afterbirth.
Of course the article was saying that while all fraternal twins are di/di, that can also occur in 20 - 30% of identical twins. It sounds like if they are mono/di though that they will most likely be identical.
Here's the question though, have they ALWAYS looked to share one placenta? The reason I ask is, fraternal twins could have started out as di/di but then the placenta fused making it look like they are sharing one and are now mono/di.
Either way, congrats!!