Justin Van Patten neatly summarises the new DateTimeOffset class introduced in .NET 3.5. In short, this new class combines the year, month, blah, blah, second storage of the DateTime with a time zone offset. The most important point to take from Justin's article is that apart from a few exceptions (which he describes), you should always use DateTimeOffset in new code instead of the old DateTime.
Naturally there will continue to be situations where you need to pass a DateTime but the DateTimeOffset will always be able to scale back to suit. It is much harder to infer time zone information from a DateTime to provide a DateTimeOffset. I'd love to see the SQL Server team introduce this data type concept to databases.