I recently gave a presentation on table aprtitioning at Sri Lankan user group meeting. Please check the site:

http://sqlserveruniverse.com/content/MeetingList.aspx

Let me clarify one point:

You can have the primary key as the clustered index too. If you do, you need to aprtition the table on the same key (I.e. ID and not on date) But you can have a clustered index on Date column while havign primary key on ID.

The date column need not to be unique.

Hope this clarifies.