This has now been updated with the new IOS 5.1
Now you can go to Settings > Notifications > Messages
You now have a couple of options here. If you click on the "Show" button, it will let you choose how many recent messages you want the phone to show if you don't answer anything, the options being 1, 5, and 10.
You also have "Alert Style". This is the "Visual Indicator" that you asked about, but this indicator is for when the IPhone is unlocked, and you are doing something with it. You can set it to None (no indicators); Banners (it shows who sent the message, as well as a part of the first line of the actual message) which appears on the top of the screen, you only need to click it to go to the message; and Alerts (tells who sent the message, approx. 3 lines of text, and asks to dismiss or reply) {Note: When Alerts is activated, you MUST select either dismiss or reply before you are allowed to continue with your work}
Other options are:
"Badge App Icon" > on/off
"Show Preview" > on/off
The one you were looking for, "Repeat Alert" > Never (this is never REPEAT, it will still ring when you receive the message), Once (again, this means it will repeat exactly once), twice, 3 times, 5 times, and 10 times. Unfortunately, Apple forces 2 minute intervals, and doesn't let you change that.
And finally, the last option, "View in Lock Screen" > on/off