Moving Web pages that have been indexed by search engines to a new URL via a 301 permanent redirect can cause serious dips in traffic once the search engines discover that the old page has moved. This is a headache that must be planned for whenever anyone considers changing the addresses of their Web pages, for whatever reason.
Barry Schwartz at Search Engine Roundtable last week highlighted a discussion at Google Groups in which Google's Adam Lasnik claimed that it only takes them a "couple of weeks for things to smooth out."
Unfortunately, although Google seems to be able to index pages within a few weeks, the past rankings for those pages are not updated in any time close to that, especially for competitive terms, without some additional effort. The primary method to speed this process seems to be gaining links from authority sites to the new URL, in a rapid fashion. Funny because that also seems to be the way out of what some call the fictional Google Sandbox.
The "trick" often used in order to try to lessen the severity of the old pages' loss of rankings is to employ a 302 redirect instead. This causes Google to keep the listing of the previous page within its index, and often in the same position within the rankings.
Google, Search Engine, 301 Redirect