I don't notice any change EDIT on my system.
How did you look in F5 mode?
Can partly confirm what Kumasasa says, but N-S seems to sometimes work and sometimes not, depending on the speed of the minecart (which would be expected - after all, the minecart can't fall into the hole).
Since minecarts are too big to fit into a 1 m² hole, it seems to me the only bug here is the different acceleration in different directions.
Confirmed in 1.5 pre-release. In fact, as soon as you are in the sort of "locked" movement, you can let go of one of the movement keys, and you still move along one axis instead of diagonally.
The walking speed seems to be that of blocking on normal ground.
Also happens when the ice is on top of soul sand.
Confirmed in Release Candidate.
This also happens when the dispenser is facing sideways, but not when it's facing downwards. Also, the block in front is necessary.
This is not happening to me...