Nice step-by-step writeup on installing Solaris 10 in a VM. Most of it also applies to Solaris Express.
A couple of things to add to my earlier notes on VMware/Solaris issues. The latest kernel patches for Solaris 10 (118844-19 and 118844-20) cause a panic on boot on some systems, including in a VMware virtual machine. Some limited details are available here. The workaround is to boot under the kernel debugger (kmdb) - type "b kmdb" when you get the "(b)oot or (i)nterpreter" prompt. You can make this persistent with "eeprom boot-file=kmdb" once you're booted (or by messing around with the boot configuration menus, but I don't recommend that for the easily frustrated).
The latest version of Solaris Express (build 27a, the one with ZFS) won't install in a VM if you use the graphical install and the default guest memory size (256MB). It hangs trying to load the X server and window system into memory. Either increase the memory (I suggest at least 512MB), or select the text install by hitting "4" when you get the question about install type. This should be fixed in a later build (to automatically fall back to the text install if there isn't enough room for the graphical install).