The old ISA spec allowed for cards to provide a BIOS of their own that could be run at one of several different points in the motherboard's startup. That's how boot PROMs work, BTW. The PCI spec also provides for this.

A lot of integrated LAN cards support net boot because the BIOS knows about them anyway. But I've recently seen ethernet cards that support a PXE environment without help from the BIOS. And you couldn't really tell by looking at the cards anyway, as there's no PROM socket at all. But there's obviously some there when you watch them start up.

Wade.