tedster - 8:26 pm on Aug 8, 2011 (gmt 0)
Get those 301s in place - you'll see some rankings reappear soon enough, as long as the newly developed site isn't full of technical problems.
And I assume you've got good internal link structure for the new site. That's a big deal. Then even more, getting some backlinks changed from the old URLs to the new ones is also a good step, even if tedious.
One thing I'm not clear about. You called this a site "migration". Are you now using a different domain name?