John Jansen (Microsoft Office FrontPage) confirmed in a private e-mail conversation what I did find out the hard way:
In the Xslt transformation of data view web parts the following functionality is blocked:
I can understand that msxsl:script is blocked, otherwise you could execute any server-side script. Why xsl:include and xsl:import is not supported I really don’t understand. Could be that they scan the script code string to be executed for the msxsl:script tag, and that handling inclusion of external code was to much work, but it makes data view programming really hard. Would be great if you could create a library of useful Xslt routines and include those in your custom xslt transformations.
Another thing I have been working on is the possibility to register additional xslt extension object like the one in the ddwrt namespace (see also http://weblogs.asp.net/soever/archive/2005/01/03.aspx), but I never succeeded.
Any ideas on extending the xslt transformations functionality in data view web parts?