I've used a consumer-grade Gigabit Ethernet switch in demonstrations of iSCSI technology and run iSCSI on a 10-Mbps Ethernet hub just for grins. But don't try this at the office or at home. Consumer switches typically don't support wire-speed connections between multiple ports, so they may drop packets without warning.
We've seen a low-end, 24-port switch with two, 12-port switch engines and a single-gigabit connection between them. But if you put your servers on ports 1 through 16 and place your disk arrays on ports 18 through 24, that single-gigabit connection will get overloaded, resulting in packet loss and a huge performance hit (or worse).
Your iSCSI SAN should use an enterprise-class, nonblocking, Gigabit Ethernet switch, such as one from Extreme Networks or Foundry Networks. And given the homicidal thoughts that run through my head when my BlackBerry buzzes at 4 a.m., spring for the dual, redundant power supplies, too.