have a look at the recent thread called "Mimic browser prior-value suggestions for text input" which references two ways to do an auto-complete field in Flex. I realise this is not quite what you are looking for, but the principles of using the local storage area to hold results could, I think, easily be adapted to your case where yuo are wanting to retain a certain number of values and (presumably) use them somewhere else in your applciation.
I would just store the usernames of those who have logged in within a database table on the server, and then on app startup populate an array with those names, or some number you desire, like the latest 10 names.
Then rather than a TextInput you could have a custom control that has a custom item editor, so when the user clicks inside the "text input" it drops down the list of latest users.
Not super simple, but that's the nature of what you are trying to do.
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