Most blade server systems can have an ethernet switch module or an ethernet pass-through module. What is the difference between both? Is the pass-though module simply "dumb" as a hub? Why would you choose one over the other?
A switch is a switch... just like any Ethernet switch.
A pass-through is just that, it's not a hub or anything. Just like a wall jack, wire on one side is passed through to the port on the other side.
If you want switching self contained to the blade enclosure (which is fairly normal, though not always) then get a switch. If have switches or want to use external switches for some reason you'll need a pass-through module so you can plug the blades into the external switch. Blades have no external connections (normally, with a very few exceptions).
I only know IBM Bladecenters, so let's stick with this.
An IBM CoppeR Pass Thru Module is nothing but a transceiver which takes the Ethernet port from the Bladecenter's midplane and lets you connect regular RJ45 CAT-V cables to another switch. There is nothing you can configure and it's simply dumb. There are also Fibreoptics pass thru modules, in case you prefer those.
A real bladeswitch (like a Cisco 3110G) connects the blade's internal ethernet ports to internal ports in that switch module. To connect the blades to the outside world, you can use the four external Gigabit Ethernet copper ports.
Another switch would be the 3110X, which gives you a single 10 Gigabit Ethernet fibreoptics uplinks.
All those switches (at least the Cisco variants) can be stackec together to behave like a single switch with one configuration.
I havent tried the other switch brands for IBM, like Juniper and Brocade.
My experience with Dell Blade (PowerEdge M1000e) is almost the same as of the one from Alex with IBM Blade. You have up to 6 free slots for different modules such as switches on the back of the Dell PowerEdge M1000e. Normally they configure the first slot with Pass-through module with master role and the other 5 slots can be managed switches with 10Gb or 1Gb ports. If you connect the external switches to the pass-through you will have network connectivity between chassis components such as blade servers and the external switches with the same configs of the external switches. But if you'd like to have your own layer 2 configs on your blade or if you'd like to have 10G links, you may need to decide for managed Switch modules for your blade over only pass-through modules.