Thanks to everyone who responded.

From the responses on this list plus a google search for iSCSI switches, it seems that the HP Procurve line is a good choice for price/performance. The Procurve 2800/2810 seems to be the bare minimum, but the 2900 series is a better choice if you can afford it, with larger packet buffers. I'll be going with either a 2810 or a 2900.