There are a few people on the DG team who can relate to this "headache" -- What happens when your client has changed URLs? How long will it take for the Google PageRank to pass on? Search Engine Watch addresses this (via SE Roundtable):
(From the original post at Search Engine Rountable...)
In a Google Groups thread Adam says:
301s pass PR and related signals appropriately. Usually takes a couple of weeks for things to smooth out, though.
I was a bit shocked by the statement of just "a couple of weeks" for 301s to "pass PR and related signals appropriately." I always thought it was a couple months or more.
SEW points out that Google may index the page within a few weeks, but that it take MUCH longer to recoup the PageRank. Link building like a rockstar seems to be the best method for speeding things along. SEW also points to the common "trick" of using a 302 error instead of a 301: "This causes Google to keep the listing of the previous page within its index, and often in the same position within the rankings."
Ultimately, post author Chris Boggs recommends riding out the 301 -- on the off-season, business-permitting. His best advice is to understand that this is a laborious process, and to plan for it in advance by using paid search and other marketing channels.