I would add just some other info to the sandbox effect.
My website went online on september 2004. Suddenly became pr5 due to good websites linking to it. The ranking for its main keywords was very very poor, not competitive keywords too.
After 6 months, on march 2005, some keywords got out of sandbox doubling the traffic.
Today (WOW) a lot of competitive keywords are out of sandbox and the traffic is very very good.
Since October 2003 the website ranked very well searching with "allintitle:<keyword>"
At the moment I think I'm completely out of sandbox (10 months....)
I've tens of websites and I can confirm the sandbox effect exists.
In october 2003 I created another website that today has gone completely out of sandbox (yes in the same day)
Today has gone "partially" out of sandbox another 3 mine websites (one created on february 2004 and another on october 2004.
Another website created on october 2004 is still completely in the sandbox. This website is in a very competetive area (travel related).
All the websites are PR5, have many themed backlinks, are updated daily/weekly and have a lot of contents.
IMO the sandbox depends mainly from the keywords (more commercial more in the sandbox) and not very much from backlinks and contents.
This is the mine :) I hope help some of you.