I can't quite remember if this is true or not, can you throw RC carts through Nether Portals? Or was that only a 1.2.5 thing?
It is actually vanilla feature, introduced in 1.3.2 IIRC. Just remember that cart can't travel more often than once every 45 seconds through one (like any other entity except player). I used cart dispensers for short routes to break and replace the cart. Also you can't ride a cart when it is going through the portal.