Is learnt UK English and learned US? Is it that simple?
I’m used to using learnt, but my US spellchecker says it is wrong.
So it's not like learnt is completely unheard of in Americal English, but learned has always been more popular, and according to the COHA timetable, the usage of learnt has been on a more or less steady decline since 1820:
(X axis: year, Y axis: incidences per million words.)
Nowadays, according to the Corpus of Contemporary American English, learnt is most popular in the context of fiction and academic publications, and least popular in newspapers:
What's more, even the British National Corpus has more cites for learned than for learnt. The stats look as follows:
It is worth noting that Merriam-Webster, Wiktionary, the American Heritage Dictionary of the English Language, and Collins English Dictionary all list learned as both a verb form and an adjective, but learnt only as a verb form.
Lastly, here's a related question: Dreamed vs. Dreamt, Leaped vs. Leapt, Lighted vs. Lit.
In American English, "learned" is the usual spelling; "learnt" is too rare.
In British English, both "learned" and "learnt" exist for the verb, but the adjective (as in "the learned professor") is always "learned". (Sometimes, especially archaically, "learnèd".)