SQL Server 2008 (Katmai) July CTP is available for download

Its no more 'Katmai' and officially called SQL Server 2008.
DOn't forget to consider the supportability of using this CTP:

• This release is not supported in production environments.
  
  
  • This release is available in English only.
    
    
    • This release is available in Developer Edition only.
      
      
      • Installation must be run from the Servers folder. Tools-only installation is not supported.
        
        
        • This release is available by download only.
          
          
          • Upgrade to the SQL Server 2008 June CTP from earlier versions of SQL Server is not supported.
            
            
            • This release has a 180-day trial period. Installations will not function after 180 days.
              
              
              • Hotfix support is not available for this release.
                
                
                • Technical support is not available for this release.
                  
                  
                  • Failover cluster instances are not supported for this release.
                    
                    
                    • There is no native IA64 support in this release. WOW64 support is for the x64 platform only.

Still we can't install SQL Server 2008 in a server where SQL Serve 2005 available with out breaking SQL Server 2005. Can we?

As suggested here it is better to use on a virtual server image than spoiling the good working instance.

Yes you can and I did, but with first CTP as for this one too there is no uninstall facility or clean uninstall to perform. Always better to use virtual image that will not disturb current working environment.
You can have a 50gb space for the virtual image in this case.