It's expensive, untested and dangerous.
The idea goes like this. Start with a coffee can foundry, possibly powered with Biodiesel or Propane. Then design a mold for what you want to build using Autocad or some variant thereof. After it's finished, send the design to a machine shop to have it built out of steel. When you receive the permanent mold, melt the aluminum and pour into the mold repeatedly and often.

The idea seems like a good one to me, I'll be testing it soon enough using common screw clamps to keep the mold tightly secure.
Has anyone here done this? The closest I've come to doing it myself is pouring into a muffin tin.
The photo below is a picture of the results which was taken from another board found here, you'll need a login, the photos are located in the forum under Machining and Tooling.

Give me a shout if you dig the idea.

UPDATE 5/24/07

It works! Using my Harbor Freight Mini-Mill I cut out pockets in two pieces of 1018 steel, each about an inch deep, and four inches across. I then cut inlets in both pieces and welded some scrap steel U channel on the tops of both mold sections to form a pool enclosure for the excess aluminum to collect inside of and stay safely contained.
Then I lit the candle on my foundry and melted the aluminum while at the same time pre-heating the molds, (connected using C-clamps) in the oven. When the aluminum melted, I poured it and it instantly solidified. After about 2 minutes of running around in a panic I cracked the mold open. The detail level is incredible. Impressions made in the mold with a fly-cutter can be seen in the casting. The casting is bright, shiny, and seemingly devoid of any burs usually associated with unfinished aluminum castings.
I'll provide photos later of the test mold and casting.