Three Boys Music Corp. v. Michael Bolton
212 F.3d 477 (9th Cir. 2000) - full text opinion
The court notes that 129 songs called "Love is a Wonderful Thing" have been registered with the Copyright Office. You can search the registration database online to get more details. You'll see that both Michael Bolton and the Isley Brothers have registered a work under that title.
You can listen to both songs and compare the Isley version (Real or Windows Media) with Bolton's (Real or Windows Media). Here is another page with the music clips as well as the 9th Circuit's full opinion.
As a side note, Jimi Hendrix began his music career as a back-up guitarist to the Isley Brothers in 1964.