A lot of people already know this but on July 14, 2011 it was announced that Content Database Sizing has been updated http://technet.microsoft.com/en-us/library/cc262787.aspx#ContentDB.
There is still recommendation to keep content databases at 200GB. Why? This recommendation is still a good recommendation because if you need to back-up and restore a database very quickly, you do not have move around large back-up files. Plus if you have an Information Architecture that drives content to specific site collections (with dedicated content databases), you will be much more agile to requirement changes, upgrades, etc. One big massive content database is an indication of poor planning and governance.
The first new recommendations in this article is that 4TB of data can be stored in a content database. There are some parameters around this recommendation that you should read.
The second new recommendation is that there is no explicit recommendation on sizing for document archiving scenarios. However there are some very specific recommendations made in here to support that scenario – so review http://technet.microsoft.com/en-us/library/cc262787.aspx#ContentDB. This is really important for Records Management solutions. On this specific point, it would also be good to review the “Extremely large-scale content archive” section in http://technet.microsoft.com/en-us/library/cc263028.aspx. When going down this path you will need to have Remote Blob Storage and FAST to support this solution architecture.
Finally SQL Server tuning is fundamental to your success for managing large content databases. Here is a blog that I wrote on the topic - http://www.astaticstate.com/2010/12/sharepoint-2010-high-availability-with.html