There is no way to merge iTunes accounts. I've seen some wishful speculation in some iOS 5 articles, but I wouldn't hold my breath.
The Airport (Express or Extreem) routers have USB ports for adding printers to the network, but there is no way for an iPad to directly access these printers. You need a computer, OS X or Windows, to use a printer t his way. Once you've done that there are a couple of ways the iPad can access a printer through your computer, but the computer must be running and on the same network as the iPad.
AirPrint is different from Airport. You need either an
AirPrint compatible Printer or a way to get your computer to pretend to be an AirPrint printer and share it's printers. Miket5au has a good link to the free way for Windows. Two pay for options with a few more bells and whistles are FingerPring (for Windows or Mac) and Printopia (for Mac).
The other way to get print capabilty from your iPad is with an app like Print n Share, or PrintCentral. These will work with many wireless printers, or can work with almost any printer using a computer program. The upside is that if you have a compatible wireless printer you won't need to by an AirPrint compatible printer. The downside is that you can not use it to print directly from apps that support AirPrint.
Use this solution if you have no desktop computer, or rarely have it turned on, or move from place to place that has compatibel printers. Otherwise the solutions that have your computer share it's printers is the easier to deal with.
Easiest, of course, is an actuall AirPrint compatible printer, but most of us don't want to buy a new printer just for the iPad.
