I'm running MIUI 2.5.25 ICS on my SGS2 and using the stock lock screen. This happens on a clean install before any apps have been installed or any settings changed. When I get a text, an icon shows up on the notification bar, but the SMS counter on the lock screen does not show up, and neither does the preview when holding down the unlock button. If I unlock the phone to the home screen (not straight to SMS app) and then re-lock the screen, the SMS counter is now updated and long pressing the unlock slider button shows the message preview. I've noticed this on the last few builds, but it has not always been this way. I believe the SMS counter on the lock screen should update immediately and that you should be able to access the message previews without first unlocking the phone.