One of the biggest additions in iOS 5 is Notification Center--a pane that holds your most recent notifications (among other things)--which you can pull down from within any app. I've been using the iOS 5 beta since jump, and constantly find myself inside Notification Center. The granular, app-specific controls are fantastic (though of course I, like Ben, have a few niggles, which I'll outline in a future post).
Regarding iOS' SMS app, a few years ago I wrote:
The main thing I'd like added is the ability to mark text messages as unread. I receive a ton of texts, sometimes in quick succession and from multiple people, and don't always want to respond to them right away (much like email), but do want to respond to them at some point. The lack of mark-as-unread is most annoying when I receive a text message while inside another application; when this situation arises, a modal window appears with the text message, and gives me two choices: "close" and "reply." If I choose "reply," I'm shuttled out of the current app and into the SMS app; "close" kills the semi-transparent pop-up and marks the message as read. These limited options mean that if I receive a text message while doing something else, I either have to respond to it immediately, or run the risk of forgetting that a response is due.
OK, so what does that have to do with Notification Center?
Within Notification Center, each app's notifications are grouped together and are cleared either by clicking the circled "x" associated with the app or by launching the app (usually). The rub here is that cleared or uncleared, the SMS app's unread badge is not affected; this means you can read your texts from within Notification Center (and/or the lock screen and banners/alerts if you have "Show Preview" turned on), without ever disturbing that subtle reminder you've at least one message to which you need to attend.
The only thing you need to watch out for is "Slide to reply"--which appears on the lock screen when you receive an SMS while the screen is off--because this will take you directly into the SMS app, and subsequently mark the text as read.
I have "Show Preview" turned off, and so I need to jump into Notification Center to actually see the content of the SMS message(s). When I receive a text I want to look at immediately, I hit the power button to toggle the screen on/off, "Slide to unlock" and then swipe down to invoke Notification Center.
With this method you can review (and clear) your text messages without having them marked as read, and then respond to them when you feel like it.
(Please don't tell my friends I do this.)