Denny.NET has a post describing how to generate a SQL case statement in LINQ to SQL.  His example is:

var gimmeh_gimmeh =
from t in db.sc_Timeslots
where (t.starttime.Minute % 5 == 0) || (t.starttime.Minute % 10 == 0)
select new
{
  t.id,
  t.starttime,
  interval = (t.starttime.Minute % 5 == 0) ? "5 Minutes" : "10 Minutes"
};