There are three good resources for understanding the new data warehouse in ITM 6.1.
1) A Gulf Breeze TDW 2.1 Overview.
This is a TUG presentation that is a high level overview of the detail and summary tables and how they are stored. This is a good first place to start.
2) Chapter 4 of the Getting Started with IBM Tivoli Monitoring 6.1 on Distributed Environments.
This has a very detailed explanation of all the detail tables and summary tables.
3) ITM 6.1 / TDW 2.1 Warehouse Load Projections spreadsheet on the OPAL site
This is a spreadsheet for estimating size requirements for the ITM 6.1 DW.