So I called Sprint, and cancelled my order. This was not easy; I had to fight my way through their 'phone system and it took a while, but it was done. Then I called Handspring, and inquired about their upgrade program. They were all sold out! Oh no! I explained my plight to the Handspring customer service rep, and he took pity on me, and I eventually did get my upgrade order in. Whew!
So I expect to get my new Treo in about a week. Stay tuned.
[ 10 minutes later: Okay, you're not going to believe this. ]
As I was blogging the item above, sitting next to me on my desk was a received package. I figured it was from Amazon - they usually are - but no, it was from Sprint. It was my new [and now cancelled] Treo 600! I am not making this up.
It looks just beautiful. The fit and finish are superior to those of the Treo 300, and the size is noticeably smaller (see pic). The keys are closer together but actually easier to hit, because of their shape, plus there's that new little cursor control. And of course the 600 has a camera (!) and an SD slot (!), while the 300 does not. Finally the 600 is not a flip phone, which is a good thing; note the broken hinge on my 300...
So now I have to return it (sigh) and wait for the one from Handspring to arrive. Stay tuned.