Thinking $500 for a USB-GPIB interface is highway robbery I bought a cheap Chinese clone of the Agilent 82357B for $125. The Beiming Technologies S82357:

It worked for about 2 minutes (albeit poorly, wasn't able to get proper IDN strings) and then died. Luckily the Chinese seller was kind enough to issue a prompt refund, and I get to ship the thing back today. In the meantime I've decided to up the GPIB game and bought a National Instruments GPIB-ENET/100 instead of another USB dongle:

This looks to be an older version of the GPIB-ENET/100 product NI currently sells, but it should still be alright. Agilent also makes GPIB-Ethernet bridges, but they have the nasty habit of dropping support for them in their drivers. So screw that rat race, NI can get my money.

This LAN-based idea is much cooler than a USB or PCI adapter. It's not just Ethernet that it talks over, but it does full TCP/IP. It means you can route to this interface from anywhere; control your spectrum analyzer remotely when it's on a mountain, or whatever! Can even run the instruments over WiFi from a laptop. Hurray for not being tied down. None of this comes cheaply though, as the GPIB-Ethernet bridge is $425. Still a fraction of the $1,150 MSRP though.

The last advantage should be better compatibility with LabView. I've read several reports now of Agilent interfaces being quirky when used in NI land.