It took me 20 mins to work out why my CAML was working against 1 environment and not another.
It turns out because I was using the IN operator and that’s new in 2010 and not available in 2007.
This new operator is a god-sent, because no longer will you have to do multiple nested OR clauses.
Here is the link to MSDN for this operator: http://msdn.microsoft.com/en-us/library/ff625761.aspx.
Bernado Nguyen-Hoan's Blog - Coding Stories from an IT Mercenary 
Hello,
I use IN operator in XsltListViewWebPart XmlDefinition on SP2010. It returns following error:
‘Error parsing CAML query – unknown operation’.
Do you have any idea why?
Here is my XmlDefinition:
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Hi Martin,
Appears the XML didn’t come through properly in your comment…