So I did a few tests based on the thread replies here, and here's what I found.
First, a few parameters and setup:
- My iPhone 4S is already on a Wi-Fi network, internet connection is fast and stable.
- I'm judging whether photos are on my Photostream based on whether they appear in my "My Photostream" album
Do photos automatically get pushed to Photostream when I ...?
1. Unlock iPhone, open Camera app, take photo? YES
2. Slide up Camera from lock screen (but when iPhone wouldn't have asked me for Lockscreen password anyway)? YES
3. Slide up Camera from lock screen (while iPhone is password locked)? YES, but inconsistent
In scenario #3, it *****, but I managed to get those photos to show up later on, simply by shutting down the Camera app (double-press Home button and flick the Camera "card" away). I guess it's fixed now?