Here are what the base strings look like:
Working, callback = oob:
POST&http%3A%2F%2Fapi.twitter.com%2Foauth%2Frequest_token&oauth_callback%3Doob%26oauth_consumer_key%3Dxxx%26oauth_nonce%3D351f72abc39ea9794935d7def68f34d0f9187eeb%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1319214477%26oauth_version%3D1.0
Not working, callback = oauth://callback
POST&http%3A%2F%2Fapi.twitter.com%2Foauth%2Frequest_token&oauth_callback%3Doauth%3A%2F%2Fcallback%26oauth_consumer_key%3Dxxx%26oauth_nonce%3D4238935ba05672b1ef828f98fa95f666cf112bc3%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1319214531%26oauth_version%3D1.0