[joe wanders in with more boring facts...]
"Different from" is the construction that no one will object to. "Different to" is fairly common informally in the U.K., but rare in the U.S. "Different than" is sometimes used to avoid the cumbersome "different from that which", etc. (e.g., "a very different Pamela than I used to leave all company and pleasure for" -- Samuel Richardson). Some U.S. speakers use "different than" exclusively.
Some people have insisted on "different from" on the grounds that "from" is required after "to differ". But Fowler points out that there are many other adjectives that do not conform to the construction of their parent verbs (e.g., "accords with", but "according to"; "derogates from", but "derogatory to"). (italics added)
The Collins Cobuild Bank of English shows choice of preposition after "different" to be distributed as follows:
"from" "to" "than"
----- ---- ------
U.K. writing 87.6 10.8 1.5
U.K. speech 68.8 27.3 3.9
U.S. writing 92.7 0.3 7.0
U.S. speech 69.3 0.6 30.1
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