Like sw1tcher says, just get it now if you need it. You can wait forever in case of updates, but if you buy it now you'll get even more use out of it until its time to upgrade. At the end of the day, no matter when you buy sooner or later a new revision will come out, but the 1.67 PB's are fantastic machines.. once you get one you'll not be able to live without it
Cant see there being any significant updates before the switch to intel.
It depend on your need. If you are willing to wait until September, why not. Apple may want to capture the back to school crowd. Still not sure when the Intel Mac's may be available. With Apple's releases lately an update could occur at any time.