Have anyone seen this message before, when accessing the official twitter app?
"(null)" would like access to twitter accounts
I have an iPod Touch with iOS 5, not jail-broken.
Is it a virus?
Twitter has been integrated into iOS 5. To view the authorized apps connected to your account, go to Settings > Twitter. Any app that has the ability to "tweet" will request access to your Twitter credentials (Safari, Photos, for example).
The error message you are seeing is a bug in the Twitter client. It should read: "Twitter would like access to twitter accounts." Allowing it access means not having to sign in using the client. Your credentials will be pulled right from iOS.
There are no viruses or exploits in the wild for any handheld Apple device. Furthermore, such exploits would require the installation of a unscrupulous app, something which Apple's App Store protects against.
You can read more about Apple's iOS Twitter integration here.
It is a bug in the Twitter app, not a virus.
I would report this as a bug to Twitter.