I noticed my 4's lock screen has full brightness, or at least much higher than I have it set to. I don't remember this happening on my 3G in 3.0, and I even got it to work with iOS 4 for a short time and I didn't notice it. I just took a lock screen screenshot (so I had the same image) and opened it, and the lock screen was definitely brighter. Was this always the case and I just didn't notice? Anybody else with 4's notice this?
I thought it might take a second to use the light sensor and adjust (I have it set to ~45%, auto-brightness on) but it doesn't change if you leave it on the lock screen.
This was something new that started with the 3GS if I remember. I don't recall it happening on my 3G. It has carried over to the iPhone 4. The lock screen appears to be full brightness. After you unlock, the screen shows up to whatever brightness you have set.
Rather strange but whatever.