Sometimes new law is very old. According to the OED, the term plagiarism was first applied to music in the Monthly Magazine of 1797, when a composition was described as “the most flagrant plagiarism from Handel" (The Imaginary Museum of Musical Works by Lydia Goehr, OUP). Since then a pantheon of musicians have been accused of lifting melodies – from Jerome Kern (Fred Fisher Inc. v. Dillingham, 298 F. 145, 1924.), George Harrison (My Sweet Lord costs over half a million 1981 dollars to settle: Bright Tunes Music Corp. v. Harrisongs, 420 F. Supp. 177 (S.D.N.Y. 1976)), Mick Jagger (co-author with KD Lang), Stevie Wonder, I Just Called, and this week – Men at Work. It's up at Austlii as Larrikin Music Publishing Pty Ltd v EMI Songs Australia Pty Limited  FCA 799 (30 July 2009).
For lawyers, there's a wonderful digital archive at Columbia. The archive is very eclectic – from Izzo (H.O.V.A.) by Jay-Z, through You'd Be So Nice To Come Home To by Cole Porter, When the Red, Red, Robin Comes Bob, Bob, Bobbin' Along, We Are The World, to Ready to Die by Notorious B.I.G. All have been litigated. Tip of the hat to the bloggers at UofA for a Morrissey discussion. In China the band Flower scored a hit with their popular song "Xi Shuashua" at the end of 2006, which does somewhat resemble the Japanese band Puffy's song "K2G Rushing to You". The use of the archive can be well seen in its page on CCC leader, John Fogerty's travails
Originality in music is of course relative: Your riff or mine? A Sixth Circuit decision tried to sort out the real from the fake. But no music can really claim to be original. Peter Gutmann. American Lawyer 27.9 (Sept 2005): p95(3).
There are only a handful of Canadian cases:
The most famous is about Hagood Hardy's The Homecoming, which Justice Douglas Carruthers once described to me as the most fun that he had had in a courtroom. In Drynan v. Rostad (1994) 59 C.P.R. (3d) 8, CarswellOnt 2217 Ontario Court of Justice (General Division) [Small Claims Court] the case turned on a CBC theme. In July 1989, Drynan composed a musical work entitled Filion Family Welcome and recorded it on an audio cassette. A year later Rostad composed Here We Are On The Road Again, which became the opening theme for a CBC series.
The tests are fairly clear: Francis Day & Hunter Ltd. et al. v. Bron,  2 All E.R. 16, and adopted by Denault J. in Grignon v. Roussel et al.  38 C.P.R. (3d):
It is well established that to constitute infringement of copyright in any musical work, there must be present two elements. First, there must be sufficient objective similarity between the infringing work and the copyright work or a substantial part thereof for the former to be properly described not necessarily identical with, but as a reproduction or adaptation of the latter. Secondly, the copyright work must be a source from which the infringing work is derived. It need not be the direct source. There must be a causal connection between the copyright work, i.e. the copyright work must be shown to be a causa sine qua non of the infringing work.
Belgium seems to be developing a reputation as a litigation centre in the area. Last August, the Brussels Court of Appeal rejected a plagiarism claim about US rapper Eminem's song "Cleanin’ Out My Closet", Under Belgian law, plagiarism must be assessed on the basis of the perception of the disputed works by a general audience and claimants must demonstrate substantial similarities between the works. But Michael Jackson's hit song "You Are Not Alone", written by singer-songwriter R Kelly, was banned from Belgian airwaves after the same court held that it was a tad too similar to an earlier song by Belgian songwriters Eddy and Danny van Passel. Amazing that the Great One stayed up at night listening to Belgain pop songs. Of course so did Madonna whose song "Frozen" contained elements of an earlier work by a Belgian composer.
Thanks to Carissa L. Alden in the Cardozo law Review for pointing to a great analysis by David Bollier in Brand Name Bullies which he shows just how rich the borrowing has been, even in the days before sampling. “‘Good Night Sweetheart’ (1931) is based on themes from Schubert’s Symphony in C and Liszt’s preludes”); “‘The Lion Sleeps Tonight,’ also known as ‘Wimoweh’— recorded by the Weavers in 1952 and the Tokens in 1961—is based on a traditional African song”); and folk songs based on religious tunes (“[T]he melody of [Bob] Dylan’s first big hit, ‘Blowin’ in the Wind,’ was based on an antislavery spiritual, ‘No More Auction Block’.
Paul Goldstein in Copyright: Principles, Law and Practice also cites a brief from a musical infringement case which noted that “the following well-known compositions all contain five to seven consecutive pitches in common with each other: As Time Goes By; The Star Spangled Banner; O Holy Night; Three Blind Mice; God Save the Queen; and Stranger in Paradise.”
Steven Fox, Program Notes to “Gemütlichkeit von Salzburg” (Oct. 23, 2007) shows how Michael Haydn cribbed from Mozart in his Requiem pro defuncto Archiepiscopo Sigismundo, MH155 “how different the times were, when what might be considered plagiarism today was nothing but the greatest honor one composer could pay to another in the 18th century”).
Men at Work are in good company.