There is no difference between the two forms. Both versions are correct.

The word "dreamt" is more likely to be used in Great Britain and "dreamed" is used more in the USA. Both are completely interchangeable.

I use "dreamt" for past perfect, dreamed for any past. But I agree that they are interchangeable. Oxford English has never heard of "dreamt".

Just in case this list might be of any use to anyone around here, even if this particular thread is ancient:

http://www.ssdd.uce.ac.uk/learner/grammar/les...
(a list of "Less common irregular verbs", whatever "less common" is suppose to mean).

@delicately chiselled features:
Here is what the Oxford Dictionary has to say about dream:
dream
• noun 1 a series of thoughts, images, and sensations occurring in a person’s mind during sleep. 2 a cherished ambition or ideal; a fantasy. 3 informal someone or something perceived as wonderful or perfect.
• verb (past and past part. dreamed /dreemd/ or dreamt /dremt/) 1 experience dreams during sleep. 2 indulge in daydreams or fantasies. 3 contemplate the possibility of: I never dreamed she’d take offence. 4 (dream up) imagine or invent.
and this:
What is the only word in the English language that ends with the letters -mt? The answer is dreamt (past tense and past participle of the verb dream, in British English). If you were being pedantic there's also adreamt and undreamt.

Which Oxford were you looking at?
I agree otherwise with Katharina - dreamt is BE, dreamed is US