The 90EX can act as a Master controller but when set for wireless (Master) it will not fire. So the 90 cannot be fill light for a [Canon-style]wireless setup. I was bummed to read that in the Canon manual for the little unit. It says"optical" trigger but I believe Canon means IR only. The sensors on the other units need to be pointed back toward the 90 and the camera, unless you are lucky enough to get consistent bounce of the IR while shooting inside where the walls provide an efficient enough scatter.
I bought the Speedlite 270EXII a little while ago and it has a "Slave" setting only so cannot act as a controller while on the camera. That is annoying as they added this so soon after in the 90.
Good to know that a cheaper speedlite can be used as master although via optical mechanism only. Most of the time the master is disabled anyway (very low to no power) so if the 90ex is always disabled, not really too much of a con.
Bodies: Canon T2i gripped, EOS 5, EOS 100
Lenses:EF 24-105 F4L, EF 70-200 F4L, EF 50 F1.8, EF-S 18-55mm, EF 28-80 USM
Other : 540EZ, 320EX, 430EX II, Yongnuo triggers & strobist stuff