Im having the same problem as the original poster.
i have an iphone 4s. photostream is turned on on both my pc (win 7) and the iphone.
photo's taken with the iphone go to the camera roll, but NOT to the photostream folder on the phone, and never show up on the PC in the photostream folder.
i have tried just about everything i can think of.
nothing has worked.
photostream was working on the phone the day i got it, but not ever since.
on the day that i got the phone, i took three pictures.
all of these went to the photostream folder on the phone, and then to the PC via the cloud. no photo taken since goes to the photostream folder, just to the camera roll.
im guessing this is a big glitch, and we will see 5.0.1 very soon.....
Jason.