Download and run usbcopy script.
Sample output:

(alex@milax)$ pfexec ./usbcopy milax03.usb
Found the following USB devices:
0: /dev/rdsk/c4t0d0p0 953.5 MB Kingston DataTraveler 2.0 1.00
Enter the number of your choice: 0
WARNING: All data on your USB storage will be lost.
Are you sure you want to install to
Kingston DataTraveler 2.0 1.00, 953 MB at /dev/rdsk/c4t0d0p0 ?(y/n) y
umount: warning: /dev/dsk/c4t0d0s0 not in mnttab
umount: /dev/dsk/c4t0d0s0 not mounted
Copying image to USB device
7744+0 records in
7744+0 records out
126877696 bytes (127 MB) copied, 57.9422 s, 2.2 MB/s

real 0m57.950s
user 0m0.011s
sys 0m0.304s
Installing grub to USB device /dev/rdsk/c4t0d0s0
Completed copy to USB
(alex@milax)$

Note: Version 0.3.1 include usbcopy script.
Good article from Bernd Schemmer : “OpenSolaris and Linux on One USB Stick for Dual Usage”

Windows users: you can now try OpenSolaris Live USB Creator (Windows/.NET).