simple tutorial

1. get hardware information
1.a plug your unsupported printer into an unused USB port and turn it on
1.b open //applications/utilities/Apple system profiler
1.c go to the 'Devices and Volumes' tab
1.d you'll find one USB bus or more, depending on your hw configuration. Search among the boxes, until you find one that has '(Epson)' (or anything similiar) written into it
1.e click the triangle next to that box
1.f take a look at the 'product id' string. after the numbers, you'll find a semicolon and a string like 'Stylus Scan 2500' (no quotes, your probably will read 'Stylus Photo XXX' or something like that)
1.g carefully write down this string. it's your printer's id, and the key to it all.
1.h if you have problems, you can safely assume your printer's id is the obvious choice (eg. 'Stylus Color 750', no quotes)

2. more hardware information
2.a go to epson support site (it should be www.epson.com/support)
2.b go to the support page for your printer, have a look at the documentation and download the .pdf document with your printer's tech specs (you can skip these two points if you, unlike me, save your printer's manuals. the information is also there)
2.c now take a list of supported printers from Epson (find it somewhere under the 'MacOsx support info' link ). you need to find a supported printer that has tech specs very similiar to your. a good indicator is resolution (the two printers, yours and the supported one, should match all the listed resolutions not just some of them), then the number of holes in the printer head. obviously you should match the printer's family (a Stylus Color for a Stylus Color, a Stylus Photo for a Stylus Photo...)
2.d if you are unsure you can always try ALL of the supported drivers, but'll be a lenghty process...
2.e (submitted by Justin Wall) another clue is given by your ink cartridge package. take a look at what other printers are compatible with your printer's cartridge, and use those drivers as reference

3. now with the software
3.a if the matching printer you have chosen is directly supported from 10.1, you have already the drivers in the right place
3.b else it's time to go again to Epson's, and download the drivers. follow their instructions to install.

4. get your hands dirty
4.a download a copy of 'HexEdit' (do a search at www.versiontracker.com)
4.b go to //library/printers/epson
4.c there you'll find the drivers. the one you need is called like your matching printer (eg. SC740.plugin for Stylus Color 740)
4.d ctrl-click on the driver, and select 'show package contents' from the contextual menu. a new window opens. Michael C. Haller warns us that a .DSStore invisible file is created when you open a directory. This files could potentially lead to nonfunctioning drivers or printcenter malfunctioning. As it is invisible, the file should be deleted via Terminal, or using TinkerTools utility (have a look at versiontracker) to make all invisible files visible.
4.e open the 'contents' folder in the new window
4.f open the 'MacOS' folder
4.g you'll find a document called like the short name of the matching printer (eg. SC740 for Stylus Color 740). make a copy of this document and put it in the desktop. make another copy and put it in a secure place.
4.h open the copy on the desktop in HexEdit (via File->open menu). don't be intimidated by what you see
4.i do a find (cmd-F), and search for your matching printer's model number (eg. '760', no quotes for Stylus Color 760)
4.j take a look at the right coloumn (the one with text, and not just numbers). You'll find the number you've searched for.
4.k you have to change the name of the printer three times. the first is when you see 'PMPrinterLongName' just before the number, the second is when you see 'PMPrinterShortName', the third is when you see 'MODEL:'.
4.l make the changes by selecting the number in the right coloumn, then write the number of your printer in place of the number of the matching printer.
Stephane 'Alias' Gallay has pointed to me that you have to change only the three occurrences mentioned, otherways bad things can happen (infact you can loose your printer options, among others).
4.m save the changes, and put back the file in its original place - overwrite with confidence if you've saved another copy as suggested in 4.g
An important note from Harry Werman: the owner/group of the file you used has been changed from system/admin to username/staff. You may want to change the privilege back to the original setting (using chown/chgrp terminal-based utilities)
4.n the driver is ready. go to the print center (in //applications/utilities/). two things can happen:
- your printer (connected and turned on) is still listed as unsupported printer. something went wrong during the tweaking process. restore the original copy of the driver, go back to 4.g
- your printer is now listed with its name.
4.o go to any application (preview will do), open a document and try to print.
- if the document prints, the hack works! happy printing
- if it does not, probably the matching printer you've chosen is not compatible with your printer. try with another model and step back to 3.a