Most likely, "dream" is in transition between irregularity and regularity. I would expect that 200 years ago "dreamt" was nearly universal, and (although it's harder to verify) that in two hundred years time "dreamed" will dominate.

But, as everyone else said, read the Pinker. Best titbit from it: only 2% of German nouns have regular plurals. Indeed, there are irregular methods of plural formation that are much commoner than the regular one.