What should be callback URL?

Try: http://example.com/twitter/oauth

Taken from #493784: Information on "Sign in with Twitter".

This does not work - I've tried on 2 different sites!

On the Twitter app settings page, after I submit the form with the Callback URL in this format (copied and pasted from /admin/settings/twitter), I get this error msg:
Not a valid URL format
You might be wanting to put a custom protocol in your callback url.
Please leave this field blank and email us at api@twitter.com with your desired callback url and this application's consumer key and we'll add it for you.

I did e-mail them even.. their answer was:
The OAuth standard allows you to dynamically define your callback URL in the request_token step of the OAuth handshake. Instead of registering a custom callback with your application settings, please send the oauth_callback parameter when you request /oauth/request_token . Your desired callback URL will work correctly when used this way. For more information, please see http://dev.twitter.com/pages/auth#request-token .

Please help...

Ohhh - seconds after I've found out I had a space at the end of the URL.

In FFox, copying the Callback URL from Drupal into that Twitter settings form kept a blank space at the end, and that's why Twitter did not accept it!

Guess they should use trim() ... and I should be more attentive!