they are both technically correct and interchangeable. However Americans seem more likely to use Learned and Dreamed while British are more likely to use learnt and dreamt.

This is an interesting question... (I'm an American) I might be totally off here, but these are my impressions: Learnt -- I think this is infrequently used in American English. It sounds old and stuffy. In the US, I think learned is much, much more common. However, dreamt -- I think is actually as common as dreamed... Last night, I dreamt I won the lottery. Sounds right.