Yes you can have sql server 2005 and sql server 2008 on the same cluster, this came up while turning an active / passive cluster into an active / active cluster to make better use of resources.  [They say not to use the terms active / passive any more, but it'll take me until sql 2010 to get correct my jargon].

See the section on ‘Multiple SQL Server Versions Within the Same Windows Server Cluster (SQL Server 2000 and 2005 with SQL Server 2008)’ in the link below:

http://download.microsoft.com/download/6/9/D/69D1FEA7-5B42-437A-B3BA-A4AD13E34EF6/SQLServer2008FailoverCluster.docx

I've uploaded the document here,SQLServer2008FailoverCluster.docx (3.75 MB), as I have a tendency to need to support things longer than Microsoft makes them available.

I did run into an issue with a necessary patch for the operating system that the installer did not catch until I'd filled out all the information, it then through a warning on the missing patch and would not let me proceed, of course the damned patch requires a reboot so I had to start all over.

This patch was for windows 2003, filestream support... of course I did not select to enable filestreaming, but no matter, it insisted on it.   At least it (the installer) found the issue before it started, as I really hate bombing or failed installations, but it was still a pia to go back, down load the patch, install it, reboot it, and then restart setup....i thought the whole point of the prerequisite checks was to find pre-reqs !  http://support.microsoft.com/kb/937444