A student saw “wilful” used in an opinion, and asked whether it was a typo. How things have changed in a few decades! Here’s a Google Ngrams graph comparing the use of “wilful” (blue) and “willful” (red) in Google’s American English sources:
“Wilful” was once the only common spelling (and still remains the dominant spelling in British English, again according to Google Ngrams). But then things changed, and now “willful” is considerably more common. Indeed, a quick Westlaw query suggests that “willful” is 10 times more common in 2011 court opinions. It’s thus probably wiser to use “willful,” unless one knows that one’s audience (say, a judge) has a contrary preference; using the more common spelling is more likely to convey your message without needlessly distracting the reader.
Interestingly, the first two references I found for “wilful [sic]” in court cases were in 1962 and 1963, though in those years judicial usage was nearly evenly split between “wilful” and “willful.” Those references were the only such “sic” references until 1971, but it the last few years, there have been more than 10 “wilful [sic]” references in court cases per year, which further reflects how dominant “willful” has become.