One problem: The Magento installation had 9000 products, with names, descriptions, SKUs, and other fun attributes. Spree has no built-in import or export function. Do I want to enter all this information manually? Fuck no.
After some googling, yahooing, and asking jeeves, I found a Google group discussion, took some of the example code, tailored it to the CSV format of a standard Magento export, and ended up with this ugly hack:
If you don't have the FasterCSV gem installed, install it before running this import script:
gem install fastercsv
Use the Rails runner to run this script so it has access to the database:
and you're done.