After finally sorting out a very intermittent alternator fault in my car, in my google travels I found some modern alternators use square and rectangular section wire to increase the copper density of the windings with the end result being a physically smaller and lighter alternator that can put out more power.

Anyone seen or heard of this being used in any of the current crop of "top of the line" electric motors?

Although I can't imagine skin effect coming into play at the frequencies an alternator works at?

AFAIK that's not being used. Reason is that all commercially available motors I know use multi-strand winding and the individual strands are more or less chaotic.
Rectangular wire is more suitable for single strand applications. It has been used in DIY motor projects.

Yeah, I was surprised at the lack of turns in my alternator when I pulled it apart, I still have one with good ol' circular cross section wire.

It's pretty bulky for a 3S 55A inrunner :-)