IPad – ny way to print directly from the iPad to the wifi-connected printer

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.

Go to System Preferences on the mac, click on the sharing icon under Internet and Wireless. Highlight Printer Sharing and select the printer from the Printers: check box. Make sure Users "Everyone" is set to "Can Print"

You should be able to see the share from the other computer.