There is also a USB image. You can use it, quite conveniently, with dd tool on Linux or with OpenSolaris USB Live creator on Windows written some time ago by Hiroshi Chonan.
To write it to USB stick with dd, you have to prepend a special header, which you can download here. Just do:
cat 2G.header oi-dev-148a-x86-20110223-1.usb >> oi-dev-148a-x86-20110223-1-header-prepended.usb
dd if=./oi-dev-148a-x86-20110223-1-header-prepended.usb of=/path/to/your/usb/device (ie. /dev/sdb)
Of course, with OpenSolaris based distributions, you can use /usr/bin/usbcopy from pkg:/install/distribution-constructor
And so it happened. OpenIndiana has now published illumos based authority and isos.
Authority: http://pkg.openindiana.org/dev-il/ - based on illumos changeset fe7962c08d1d; the latest changeset in illumos repository.