Hello Jeff,
For the symbology, I am currently going with "unique values, many fields" because the valid begin and valid end times of my polygons are each a layer attribute. Since the layer is created through a script I wrote, I could adjust the code to combine the beginning/end times into a single attribute and then change the symbology to "unique values". So I do have options there. Each polygon is intended to display as a unique solid color.
My problem then becomes how to make that value into 2 lines in the legend. We already know we can't make the legend label into 2 lines in 10.0 (and I'm hoping the improvements you mentioned in 10.1 help make this possible in some way or another). Arcpy does not provide a function that allows the legend descriptions to be edited from a script, but I'm wondering if there's a python guru out there who came up with a backdoor way to do it, or if anyone knows how to do this in C# or any other language.
For your second paragraph, I'm not sure I'm answering correctly, but you can make the item descriptions under layer properties > symbology into multiple lines manually. To get to the next line in the description box, you have to hit Ctrl-Enter.
Nick