How to write generic IEnumerable<SelectListItem> extension method

Well, you could do something like this:

public static IEnumerable<SelectListItem> ToSelectListItems(
     this IEnumerable<T> items, 
     Func<T,string> nameSelector, 
     Func<T,string> valueSelector, 
     Func<T,bool> selected)
{
     return items.OrderBy(item => nameSelector(item))
            .Select(item =>
                    new SelectListItem
                    {
                        Selected = selected(item),
                        Text = nameSelector(item),
                        Value = valueSelector(item)
                    });
}

You could pass in delegates to do the comparisons, and property retrieval. Something like this:

public static IEnumerable<SelectListItem> ToSelectListItem<T>(this IEnumerable<T> items, 
    int selectedId, Func<T,int> getId, Func<T, string> getName, 
    Func<T, string> getText, Func<T, string> getValue)
{
    return
        items.OrderBy(item => getName(item))
        .Select(item =>
                new SelectListItem
                {
                    Selected = (getId(item) == selectedId),
                    Text = getText(item),
                    Value = getValue(item)
                });
}

Then you would use it like so:

var selected = specificObjects.ToSelectListItem(10, s => s.Id, s=> s.Name, s => s.Name, s => s.Id.ToString());

In order for this to work as written, your type T will need to implement some interface which provides Name and Id properties:

 public interface ISelectable
 {
     string Name { get; }
     int Id { get; }
 }

With this in place, you can do:

public static IEnumerable<SelectListItem> ToSelectListItems<T>(this IEnumerable<T> items, long selectedId) 
     where T : ISelectable
{
    return
        items.OrderBy(item => item.Name)
        .Select(item =>
                new SelectListItem
                {
                    Selected = (item.Id == selectedId),
                    Text = item.Name,
                    Value = item.Id.ToString()
                });
}

This is required in order to use the Name and Id properties within your extension method... You could, instead, provide a different means of receiving these (ie: passing delegates), but that may or may not increase the maintenance cost in your scenario.