Basically downgrading is not supported by the SQL server software, but still you can try few workarounds ( having in mind Mike's suggestion ). What I would suggest is to use a data compare tool. For example the one that comes with Visual Studio.
You can a Sitefinity database on your 2005 server and then use data compare to push the contents from the running 2008 instance to the 2005 server. Still you may encounter some version mismatch errors on the way, but you can try also 3rd party tools to migrate your data.
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