i tried to send a minecart through a nether portal and it didn't work. i tried with me in the minecart and with an empty minecart.
Comments
I'm curious if this is even an intended feature? I know in the last update, one of the line items was "Mobs, items and projectiles now pass through [nether portals], preserving momentum". Does this include minecarts on rails? Should it work with an empty cart, or with an animal, chest, villager, user, etc. in the cart? During my brief bit of testing, where I tried to send myself in a minecart through a nether portal, I simply got stuck, and had to log out/in to get unstuck.

Related to MC-67, though that only specifies joined entities, such as a minecart with something in it.
When nobody is in it it should travel to the nether/end but when someone is sitting in the cart they should not be teleported. That's intended.

Except now it's marked fixed. It's not clear what the resolution is, whether passengers are teleported or just empty minecarts. But it is something we someone could check, right now, since 12w50a is out.

Tested with 12w50a: Only unoccupied Minecarts travel through the portal.

That is intentional.

Would you please expound upon that? Why is it intentional? Is this a technical limitation, or a deliberate gameplay choice?

Technical limitation and slightly a gameplay decision. Especially for players.
This should be reopened. I really doubt this is intentional but rather that it is hard. But just because it is hard doesn't mean it shouldn't be done. Ridable entities go through portals, other entities go through ports, but from a game play perspective there is no reason an entity with a rider shouldn't be able to also go through a portal.
Empty carts don't move at the same speed as carts with players. I'm pretty sure this isn't a bug but intended feature. Did you try placing a powered rail in front of portal?