DropDownList getting cleared on postback

Hi there!
I have a web page with two nested DDLs: the first one has some predefined values, and the second one loads differents values regarding of the first DDL selected value.
The way I'm doing it is this:
On first DDL selectedindex event, I change the datasourceID, datatextfield and datavaluefield for the second DDL (I have some SQLDataSources ready for this), and then I just call the DLL Databind method. So far, it works fine. The second DLL is properly populated and I can select whichever value I want from the second DDL. The problem is when I press the submit button (a simple ASP.NET button), the page do a postback and just then the second DDL loses all its items and, debugging, I just can see one null item inside this DDL. What can be happening here?
I've tried so far enabling and disabling EnableViewState and AutoPostBack for the second DDL and I'm always getting the same result.
Well, I hav ...