This is still an issue in both 1.5 and the 1.5.1 pre-release.
If you queue up a few minecarts like in minecraft-piston-collision.png and use the piston to push them one by one, the last one will not be pushed. There's no rail in front of the piston, that's on purpose. I still expect it to be pushable.
Yes. I'm still able to reproduce the issue in 1.7.4. I'm not the issue reporter, so I can't update affected versions.
As described in my last comment and minecart-piston-collision.jpg, all minecarts, except the last one is pushed. If it's only one minecart, it's not pushed either.
There also seems to be a glitch where the derailing minecart is seems to be in the ground (one block to low), but the piston can still push them if it's not the last one. A queue of minecarts on a slope will constantly push eachother, but the last one is not pushed by another minecart.
I have some new info: See minecraft-piston-collision2.png. The piston will always push a minecart if the yellow block is removed. Even if it's only one minecart in the queue. Space issues might be a keyword here.