Alright, since I haven't really seen anyone else detail how to do this, I figured I'd make an easy-to-follow tutorial on using minecarts to draw magma from a source such as the magma sea without use of rollers. I'm not sure if anyone has managed to do this successfully yet, but I like to call this set-up a "Pump Track." Keep in mind that this exploits the impulse-ramp glitch, and Toady may fix that during the next release.
Here's how I set it up, plus pictures:

Here is the top-level of the pump-track. The up arrows you see are all Upward SW Ramp Tracks (the impulse ramps). The exact number isn't all that important, as long as you have enough to make the cart move at top-speed. The minecart moves left from the Track Stop (which dumps south) along the impulse ramps until it reaches the trench. Since it's moving at top-speed, the wall above the trench is necessary to encourage the minecart to go down a z-level rather than derail. The second Track Stop is where I have my dwarves guide the magma-filled minecart back to the first one, where it's dumped down the staircase (which is what I was using to hold the magma I pulled out of the sea). Keep in mind that dwarves are not 100% necessary and this entire process can be automated; I simply use them in this example as an easy off-switch.

Here's the trench, one z-level below. This is where the magic happens; normally, when players manage to get minecarts to move through the viscous magma without stopping, the cart fails to fill. Here is what the ramp directions look like, since the photo isn't very informative:
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The furthest right ╝ is an impulse ramp. The furthest left ╝, also an impulse ramp, is what the minecart will be returning on.
What happens is that at the end of the trench, the minecart fails to roll up (since there is a wall blocking it on the upper z-level). The minecart then rolls backwards, turning along what was once an impulse ramp and rising up one z-level. I'm not entirely sure what all is happening on the physics-level, but I think that because the minecart "stopped" when it changed directions, it was permitted to fill with magma. Either way, the end result is a full cart of magma.

Here is where I had the minecart dump my magma; I let it run for a few seconds and already it had filled quite a bit.
And there you have it! This is the only way I've seen a magma pump track work without rollers; it's a simple and easy way to automate magma-pumping that doesn't kill frame-rate (like a pump-stack). Granted, it's a bit slower than a pump-stack, but you don't have to worry about powering it. Conceivably, you could put multiple carts on one track, but I haven't tested it. I suspect that they would interfere with one another, some how.
Anyways, what do you guys think?