I was wondering about this feature as well. Could the syntax look something like this:

public static class Extensions {
    public static object Cell{
        get(this DataTable t, int row, int column) 
            { return t.Rows[row][column]; }
        set(this DataTable t, int row, int column) 
            { t.Rows[row][column] = value; }

// usage
dataTable.Cell[0, 2] = "Hello World";

The usage would be the same, but the compiler would just have to translate this new syntax and it differs from indexing syntax.

public static TimeSpan Minutes
   get(this int i) 
    { return new TimeSpan(0, i, 0); }

public static DateTime Ago
   get(this TimeSpan ts) 
    { return DateTime.Now.Subtract(ts); }

They still look like properties.