How do you erase pictures from the Photo Stream album on an iPhone running iOS 5?
EDIT   

As of March 7th 2012 you can now delete individual photos from Photo Stream if you are running iOS 5.1 or iPhoto 9.2.2  



Believe it or not, it seems you can't delete individual photos from iOS5 Photo Stream.   It seems you'll need to wait for them to scroll off as Photo Stream just holds your most recent 1000 pics.  Deleting a picture from Camera Roll has no effect on the photos on Photo Stream.

According to this page:


  iCloud manages your Photo Stream efficiently so you don't run out of storage space on your iPhone, iPad, or iPod touch. If you have Photo Stream enabled on your iOS device, every single photo you take appears in a special Photo Stream album that holds your last 1000 photos. You can't edit or delete photos from your Photo Stream. If you want to touch up a photo or keep a favorite shot permanently, simply save it to your Camera Roll. iCloud stores new photos for 30 days, so you have plenty of time to connect your iOS device to Wi-Fi and make sure you always have your most recent shots handy.


So unless you delete it before it's uploaded to the cloud, you need to follow the procedures below.  You need to reset your Photo Stream from iCloud.com or turn off Photo Stream within iOS5 on your device.

Turn off Photo Stream in iOS5:  


Settings > iCloud > Photo Stream
Switch off and then back on to clear Photo Stream on this device
Repeat for each iOS5 device  


Reset Photo Stream on iCloud.com  


Sign into iCloud.com
Click on your name in the upper-right corner
Click on 'Advanced'
Click 'Reset Photo Stream' to remove all Photo Stream pictures from iCloud