Answering my own question, I was looking on Amazon for "pedal power supply" and sure enough, they all seem to be negative center.
That's a really interesting difference between pedal power supplies and what's "normal" for the most consumer devices.
So let this be a warning to anyone building their first pedal and hoping to re-use one of the 9V power supplies they may have lying around: Check your polarity: It's likely backwards from what the circuit expects, and you'll have to reverse the wiring when connecting the power plug.
Edit: Some good info here:
http://stinkfoot.se/archives/532
"Well, this is all Boss’ fault… before they started using the barrel type plug, almost all pedals had the 3.5mm (1/8″) mini plug instead. That plug is still used on some Electro-Harmonix pedals and the RAT, by the way. But Boss chose the 2.1mm barrel plug/jack for their compact pedal line, and others have since followed (for instance, Ibanez started using them in ’83, when the 9 series was released). Like the 3.5mm (1/8″) jacks, the 2.1mm jacks had a switch function, which was (and still is) used to switch the battery positive out when a plug is inserted in the jack. On the 3.5mm jacks, that switch function was located on the tip connection, while on the 2.1mm jacks it is located on the sleeve connection. So the positive voltage needed to go on the outside of the plug. Simple as that."