It seems that there is some confusion to how Photostream and iCloud work – so I made a lot of pictures to actually test the behavior.
My test setup is three iPhones (two 4s and one 3Gs), a MacBook Pro and an iMac as well as an iPad2.
My iMac is the system where I store all of my media so on there in iPhoto I chose to always automatically import all Photostream pictures into new events. This way I do not have to import pictures from the several iPhones anymore. On my MacBook Pro I only look at the pictures from the Photostream so I do not import into iPhoto events.
All my devices upload pictures as soon as connected to WiFi to Photostream (as part of iCloud) to sync between all six devices. This works fine and all my pictures are always up to date on all devices. Here the only issue would be if a device is offline for more than thirty days when older pictures get deleted from iCloud (and it is important to mention that this is not seen by you as the user – it is just on the server while Photostream on your device shows much older pictures).
When I hit the 1000 pictures limit my oldest pictures get deleted on all Photostreams on all devices (iOS and OS X). There are never more than 1000 pictures in my Photostream.
So if you do not import pictures automatically in iPhoto and do not sync your iOS devices camera roll with iPhoto you will lose these pictures but if you follow the same setup as I did you never have to sync your iOS devices with iPhoto anymore as Photostream and automatic importing in iPhoto will do this job for you.