Sourcing hardwarewill be the first big challenge.
Convincing people that a decent printer should cost more than a new print cartridge will be the next one. The business models of printer-and-ink sellers is obvious: lock them in with a cheap printer, then sell ink that's more expensive than oil or blood!
Openprinter won't have that business model. Ink supplies will be evaluated and purchased by the end users, with suggestions made here. The continued ink prices will be exponentially lower, but the investment will be substantially higher.
How can we create a printhead? If you know of vendors who make and sell printer parts that can be integrated into this project, please feel free to add them.
Don't just add long lists of random suggestions.Edit
A list of "printer-related hardware stuff" is of limited use. Add a description of the part and vendor you are recommending, and a reason for its addition.
Expensive, but runs the OS, and provides the IO for the printer. It is also easy to debug because it allows you to give it its own monitor and keyboard/mouse, so changes to firmware can be made locally. The beagle board is also convienient in that it is ready-made and open source. Approximate price is $149, but can be gotten for less at various auction sites. http://beagleboard.org/ Note: If possible, make sure that the code is as flexible/portable as possible. You don't want to be restricted to a board or chipset if you don't have to. Open source hardware is nice, but being able to run on various kinds of or self-designed hardware is ideal if not critical.
Arduino as an alternative?Edit
The arduino is a much lower cost device (around from around $30-$60 depending on the amount of storage space you require), is open source (http://arduino.cc) and provides a programmable microcontroller. You program it using a cross-platform development platform (Windows/Mac/*nix) which programs the chip using USB. With a "Ethernet Shield" (http://www.sparkfun.com/commerce/product_info.php?products_id=9026 from around $35-$45) this connects you to your network.
Why not just a basic AVR?
If you are going to use an Arduino then it doesn't make much sense not to design an AVR board specificall for the printer. A USB Capable AVR could be used, and the only external circuitry to get it running(other then the printer stuff) is a crystal and voltage regulation. They are programmable in C from any operating system, and can be done so over the USB port so no expensive debugging kits are necessary. They chips are from $1 to $20 each depending on the features.
96 dpi Serial Inkjet Printer Development Kit
This kit describes how old HP Thermal cartridges work, and how to control them. There is a book "Inkjet Applications – Circuits wih the BASIC Stamp 2 and SX Microcontrollers by Matt Gilliland (ISBN 0-9720159-3-0)"
that is a good reference guide on how to make a printer driving curcuits based on old HP technology.