Converting List to a DataTable and/or DataSet Extension Methods
There are a few things I notice right off the bat.


Your variable names while not bad, could be better.  For instance props -> properties.  Stuff like this makes the code easier to read.
You have the properties, why not use a foreach loop to fill the datatable (you did it in ToDataSet)
the _ prefex should be used for class variables, not local variables.
try using var when declaring obvious variable types var row = t.NewRow()
There is no error checking when you are filling the values in the data table.  What happens if it is not a class (int, double, long)?  You could force the generic to be a class by adding where T : class.
Why don't you use the ToDataTable method to create the table in the ToDataSet method?  This will eliminate duplicate code, and have 1 point of failure/modification as required.  As an aside, I would use the code from ToDataSet to create your DataTable, as it is written better.
While I applaud your use of it, I'm not sure the using syntax is appropriate here.  I would move that to where these methods are being called using (var dt = list.ToDataTable())  Having it here will more than likely cause unexpected things to happen in your code.
I would make these extend IEnumerable&lt;T&gt; as that will make them way more useful by not limiting them to IList&lt;T&gt;.


I do like your use of white space and indentation, so good job on that.  The extra indentation will be removed when the using statements are removed.  I also like the name of your methods, very clear and concise to their intent.