Ryan, I'm not sure I understand the bug report.  The following works for me:

<%@ taglib prefix="c" uri="http://java.sun.com/jstl/core" %>
<%@ taglib prefix="sql" uri="http://java.sun.com/jstl/sql" %>

<%!
  public static class Foo implements javax.sql.DataSource {
    public java.sql.Connection getConnection() { return null; }
    public java.sql.Connection getConnection(String a, String b) {
       return null; }
    public int getLoginTimeout() { return 0; }
    public java.io.PrintWriter getLogWriter() { return null; }
    public void setLoginTimeout(int a) { }
    public void setLogWriter(java.io.PrintWriter a) { }
  }
%>
<%
  pageContext.setAttribute("myDataSource", new Foo());
%>

<sql:setDataSource
  var="new" dataSource="${myDataSource}" />

---

Is the problem that the driver within the String representation of a DataSource 
you're passing isn't valid?  If so, that's an issue we're waiting on spec 
clarification for.

I beleive the problem could be related to the fact that our DataSource overrides
the toString() method.  The Error message reported shows the result of calling
that method.

Created attachment 1631 [details]
Corrects ClassCastExceptions in SetDataSourceTagSupport

Actually, it appears Jan corrected this issue already.  The ExpressionManager
was configured to evaluate as a String instead of an Object.

However, there is now an issue with ClassCastExceptions being raised.

I've atteched a patch to this bug that corrects the issue.

Fixed:
  - Applied Ryan's patch (thanks, Ryan!)
  - Also removed DataSourceUtil constructor, in which a
    DataSourceWrapper was allocated. This was out-of-date, since the
    wrapper now is allocated inside setUsingParams().