Partitioned views : how to filter on the partition key in a stored procedure ?

I am using partitioned views in SQL Server 2008 R2.

When I create a query filtering on the partition key (customerID), I get partition elimination as expected.
But when I use a variable to filter on the partition key in a stored procedure, all tables in the partitioned view are hit !  (this is visible in the execution plan).

Is this a know behaviour ? Isn't it possible to run stored procs to filter data against a partition key ?

Thank you