According to both the National Optical Astronomical Observatory (www.noao.edu) and the University of Alabama department of astronomy (www.astr.ua.edu) agree that the Sombrero Galaxy is indeed in the constellation Virgo. I usually trust university/government websites for this sort of thing - they're rarely that blatantly wrong. It's also in the Virgo Cluster of galaxies.

I also looked it up using Celestia (a free downloadable program for Mac that allows you to, among other things, look at objects in the sky). It's about midway between the center of Virgo and the neighboring constellation Corvus, which may be the cause of the confusion. Constellations of course are artificially defined things, so it's arbitrary where in the sky Virgo ends and Corvus begins.

I hope that answers your question.