Past tense: learned/learnt, dreamed/dreamt?

In my very first question, Carlo made an edit stating that learnt is a rare past tense of learn. I am accustomed to using learnt for past tense and learned as an adjective (as in He is a learned scholar.) I checked into a couple of dictionaries and they say both are valid past tenses. Do I stand a risk of not being understood if I use learnt?

In a similar vein, should dreamed be used as a past tense for dream instead of dreamt? I can't think of other such conflicts, but I guess there must be more.

Answer

Both are valid. learned is consistently more commonly used in American English:

In British English the difference has traditionally been much closer, with both being roughly equivalent, however since the 1940s, learned has become the dominant choice.