I completely agree with your point about Slicehost resizing not being a great production strategy.

Our app servers (on Slicehost) were getting hammered yesterday, so I issue a resize (1GB -> 2GB). After about 20 minutes it gets to about 97% and then stalls. In a panic I log into the server and kill all the apache2 instances. I look at the Slicehost console again and it says 40%. It actually started over. By the time it actually restarted with the resized slice it was over an hour later. The traffic was already gone by the time the resize finished.