TreoCentral.com has the best screenshot from verizonwireless.com saying:

Coming Soon: Treo™ 600 Smartphone

But, by the time I got to http://www.verizonwireless.com/b2c/index.jsp, that link was gone.

Treo rumor sites: should we believe you this time? After all, the Treo 610 still hasn't happened.

Update: I did a little further research and found a report on PalmInfocenter.com from the Morgan Stanley Semiconductor and Systems Conference. At that time, palmOne executives spoke about the prospects for getting the Treo 600 out on Verizon Wireless. The article said:

palmOne also acknowledged that the Treo 600 is currently undergoing the standard testing and approval trials within Verizon. They remarked that the company has one of the most arduous handset approval processes, which accounts primarily for the delay in the rollout. Each mobile carrier has a testing phase where they test and fine tune handsets and software that will run on their networks before can qualify the device.