To get around this, I found this post http://bohemiangrove.co.uk/tag/esxi/ which I've repeated here for your benefit:
- Download ESXi-Customizer (by Andreas Peetz) from here: http://v-front.blogspot.co.uk/p/esxi-customizer.html
- Download the drivers that have been adjusted for ESXi from here:http://dl.dropbox.com/u/27246203/E1001E.tgz
- Start ESXi-Customizer, browse to the ESXi ISO, browse to the above drivers, browse to where you want the new ISO going! Click RUN!
- Bun the ISO and jam it in your 790!!