I hadn’t considered issues of duplicate content in this case. I would usually leave the original pages up even with a 301 redirect, but mostly they disappeared and I haven’t had any troubles using a 301.

I guess there will be a period of overlap with this technique where two pages of identical content exist – so when Google applies a duplicate content filter it chooses a page from the aged and trusted domain rather than the new one. Maybe that will send some of the pages on the new domain into the supplemental index?

On the plus side it apparently does buy you some time if the 301s are going to hurt your rankings while the new domain ages. Even if pages on the new domain go supplemental or get filtered out of results by way of duplicate content, once you throw the 301 up it should return to normal and your new site will not have to compete with the old one for rankings.

Now that I think about it I guess there could be an issue here for someone with a more trusted domain to ‘steal’ your content in the transition. Pushing your new domain out of serps again? Or would the redirects help define which site is the original owner of the content?

Personally I am surprised there is any difference between the two codes whatsoever, but I can see the benefit of Google treating them in this way.

I am concerned about this approach though. You are having both old and new sites with the same content. Although the old site redirects to the new via the 302 links, effectively the old site still exists and so is duplicate to the new site.

Yes, the approach is probably working well for people. But it is certainly something that you need to keep a very close eye on.