Ever had a situation where you had to attach a database when all you had were the data files? Perhaps when you downloaded the AdventureWorks sample databases from CodePlex? Here's a script to attach a database with just the data files using CREATE DATABASE....ATTACH_REBUILD_LOG statement

USE [master]
GO

CREATE DATABASE [AdventureWorks] 
ON  PRIMARY ( NAME = N'AdventureWorks_Data', 
              FILENAME = N'E:\SQL Databases\AdventureWorks\AdventureWorks_Data.mdf', 
              SIZE = 3072KB,
              MAXSIZE = UNLIMITED,
              FILEGROWTH = 1024KB )
FOR ATTACH_REBUILD_LOG
GO

Refer my post: http://beyondrelational.com/modules/2/blogs/77/posts/15581/0176-sql-server-creating-a-database-without-the-log-backup-file-error-msg-5120.aspx for more details.