I bought one of the Nalgene lumbar packs and it arrived quickly. It has all kinds of possibilities for hydration and as a general fanny pack. It's a great start of you want some shoulder straps for a MYOG project.
It came with four 20oz/600ml Flexi Flasks, which look exactly like a Playpus to me. The caps have the same threads too, so all the accessories will interchange. Two full Flexi Flasks will fit in the bladder sleeve in the Nalgene back.
For those interested in getting a bladder for it, Campmor has 1.8 liter Platypus bladders for $10 (no hose) and they have a 1.8 liter Water Chest for $10 which includes the 1.8 liter bladder and a hose, as well as the insulated bag with d-rings. There is some confusion in the listing-- it says 1.8 liter in the listing and comes up 2.5 liter on check-out. I'll report back if it is a problem. I tried a standard 2.5 liter Platypus in the Naglene pack and it will fit if you fill it about 2/3 full and fold the bottom end over. The 1.8 liter Platypus bladder is shorter and wider than the 2.5, fitting the ratio of the Nalgene better. You can get Nalgene 1.5 liter bladders for $20, but they have the large (and heavy, IMHO) fill port.