First, g1smd pretty much nailed it in saying that 302's are unreliable. You can't predict with certainty what will happen in each case, at least not with Google (I assume it's Google we're discussing)
I wrote an article back when the problems with Google was wery much worse than they are today, and I guess that's one of the two mentioned in the original post. Credit's due where credit's due, and Google has actually improved since I wrote that article, but, well...
There are two types of problem solving: Some problems you solve 100% and then they're solved. With others you do something else, and then you've still got the problem and it's not solved but it's no longer a problem because of something else.
I will maintain that there are only those two. Google has tried to invent a third type in this case. They've made some effort at solving the problem, yet not solved it 100%, and not put something else in it's place so that the problem is no longer a problem.
Anyway, Google has cleared away the possibilities for the most serious types of exploits. No doubt about that, and that is cool. They did that pretty fast as well, that is; just a few months after my article managed to make headlines a few places.
(credit's due, etc: ... which was years after those issues were first brought up here at WebmasterWorld)
Now, Google is a big company and around newyear they started implementing a new infrastructure, which -- as I understand - should help take some of the last issues away. I'm not convinced yet, especially as the use of 302's is less widespread in SEO aware circles than it used to be, so any decrease in problem reporting may be caused by that we're less inclined to hear about problems anyway, as we don't use that stuff like we used to.
Anyway, down to the nitty-gritty
What a certain Google engineer said -- I'm not sure if it was Googleguy on this board or if it was "that other guy with the blog" -- was that (I can't quote literally, but it went something like this):
... they would try to do it like Yahoo, except they would decide in each case which was the "prettier" URL
I'm not sure "pretty" was the word or if there were more words, but it was something like "the URL that people would probably like to see in the SERPS" ... So, there are some problems here as there are clearly interpretations, and that's exactly why g1smd hit the nail on the head.
So, should you do it?
The "it" being: Use a 302 for a while as you build a new site on a new URL and get backlinks to it, and then -- as the new site starts to rank on it's own -- change the 302 to a 301.
Well, you could. I don't think you'll risk anything as either one of your two sites will be linked in Google. Perhaps not the right one every time, but they would still both be yours and the visitor would end where you wanted.
So, try a few (5-10) pages or a minor section first - 302 it and see what URL it will get in the SERPS and what rankings those pages have after a month or so. If Google seems to get it right in your case you can try 302-ing the rest.
But... you could do exactly the same thing without the 302, and you could also use a simple text link or meta redirect in stead of the 302. So... no matter if you use it or not you will have one old site ranking, and one new site building up.
Last time I moved a section of one site to a new site of it's own was end of December 2005 so it was before the Big Daddy rollout. What I did was first to physically install the pages on the new domain, and do the related minor changes (new name, obviously), and launch. After that, I replaced the content of the old pages with a single text link saying essentially "... this page has moved to (new address link) - please update your bookmarks ..."
For two weeks the old pages were left like this, with just a link. Then I 301'ed them. All I wanted was basically to make sure that a search engine had "followed" those links to the new location before I did the 301. So, I 301'ed them around January 10 this year, and the new domain was brand new; bought just for this occasion, never registered before. I had SE referrals in January on the new site. Same month as I did the 301. URL was changed in SERPS after a day or two, IIRC.
The SE referrals come on lousy terms and it's a very low volume, but the site was only three months old before I moved it, so that's no surprise and no worse than before - and clearly that has nothing to do with the move. That's just because the SEO on the site is lousy, and I can attest to that as I've done next to nothing, basically ;)
If I was going to do it again tomorrow I would not feel a need to use a 302. I would use the same tactic I used in January - basic text link first, wait for spider, then 301.
But feel free to do a 302, I'm not going to warn against that, except for saying (well, agreeing with g1smd that said it first) that it could be unpredictable. I would not think it would directly harm you as you own both sites, and traffic will end up at your place anyhow.
Perhaps it's even better than my tactic in your case. Remember that the site I moved in January was just a baby - three months old, could hardly crawl. So if it's a thousand page site with lots of good ranking pages I understand that you may want to be more careful.
And, one of the steps I took was to remove the content from the original pages before the new pages had been spidered, and that may perhaps seem a bit too risky for your liking. With a 302 you always have the backup plan of removing the 302 again, and letting the pages return to their old URLs.