there is a very complete discussion about making your own li-poly charger at:
http://www.rcgroups.com/forums/showt...hreadid=130567
the scott henion design is listed farther down in the thread, but i like the electron_head design better, its is simpler with fewer parts and less chance for error with the switches on the henion design, which of course is similar to the less expensive commercial designs with jumpers or switches.
also, it is mentioned in the other board, but the lm317 is rated for 1.5A, the lm350 is rated at 3A, and the lm1084 at 5A with an even lower dropout voltage than the 317 or 350. just in case you want to charge those 2000/2100mah packs, or parallel a couple of smaller packs and still have the quicker charge times.
im getting ready to build one of these myself with the optional charge complete indicator circuit.
anyone who is thinking about building a charger should read the rcgroups thread and/or the scott henion page about safety and such, and while it is a bit long (270+ posts) there is a lot of good info on the rcgroups thread.