We've been using Niku and now Clarity since 2001 and there still isn't any archive function. However, we did, with technical help from a CA Partner, create our own Archive database. We archived all projects that were in a completed status as of 12/31/2006 (1800+) as well as many inactive resources. Any project plans (even multiple year ones) that had time entry from 2007 remained in the Production instance. It was quite a lot of work to ensure that we had all of our lists correct of what we wanted to move out, but it has worked well. We are looking to archive 2007 data in the next few months. I would suggest that if you have a large amount of data to move, you consult with CA or an approved partner. I wouldn't do it on your own as there are many, many tables to clean up.