As of June 2012:
The rats of Aquarium Mischief are housed in a Midwest Double Critter Nation cage. I have separated the top level from the bottom level. Girls are on top, boys on the bottom. Previously the girls had the full DCN cage to themselves, and my males were in a Petco Rat Manor, but I noticed they weren't getting around much and were starting to put on additional weight. I returned them to the bottom half of the DCN and even though they still are not very active, they are moving around more since they have the additional space to do so. I have the Petco Rat Manor still available as a spare cage for when needed.
I provide hammocks, tubes, ladders, Lava Ledges, and other cage accessories to the DCN cage which I rotate out occasionally to keep their interest. At this time, only my females have access to a wheel- I am looking to add a second, for the males in the near future.