Hurray for Planet Bike.
I've liked PB for a long time, and now they've done something I've been wanted them to do for a long time: include a rack-mount with a tail-light. It's really a no brainer--if you've got a rear carrier on your bike, then the only good place to put your tail-light is the back of your rack.
The Rack Blinky 5 comes packaged with rack-mount in addition to the seatpost mount. The rack-mount is also available seperately, as it has been in the past.
Hurray for Planet Bike!
PS for the rest of us:
This seemingly insignificant change makes the lives of your mechanic and your local bike shop's inventory staff just a bit easier, and saves you a few bucks. Both are good for you!
PS for the bikey folks (i.e. not "the rest of us"):
The rack-mount is versatile, too, allowing use of either the vertical (a la Blackburn, et al.) or horizontal (a la European, Planet Bike racks, and most others) two bolt patterns. I've also successfully bolted the mount to fenders made by Planet Bike and SKS.