If your field is an NVARCHAR and you use VARCHAR parameters you will get a seek because SQL is happier to turn a VARCHAR into an NVARCHAR than the other way around.

It's worth noting that anything that involved converting a string value into another value can also have the problem.

SQL Server can take a statement such as

WHERE DateCreated BETWEEN '2011-04-01' and '2011-04-11'

and get exactly the same type of implicit conversion.