According to "Ethernet, The Definitive Guide", that feature is an
optional part of the spec.
One thing I've heard people encounter is that if they use a cross-over
cable, which probably really implies a 100BASE-TX cross-over, then the
ports only go to 100Mbps. A Gig-E rated straight through, in conjunction
with the automatic crossover feature, was necessary to get to GigE.
A lot of cross over cables only cross pairs 1-2 with 3-6, leaving 4-5, and
7-8 as straight through. Gig-E uses all four pair.
