Can I print to an Airport-shared printer from Linux? from iOS?
I found a document called the "AirPortExpressSetupGuide.pdf", which has an answer to the first part of my question:


  All of the compatible computers on 
  the AirPort network, both wireless and wired, can print to it.


So there seems to be no restriction on where you can print from.

iOS seems to have removed the ability to print to any printers except the 2 new ones HP is selling, so that's out.  Oh well.