Hi:
What you are looking for is called cascading drop downs.
I'm assuming you have your first drop down set up correctly. It is bound to a field in your main data source, I'll guess - and for the purposes of discussion, I'll call that field Contact.
So, your second drop down - for the values, you'll use the same SharePoint list as you are using for the drop down bound to the Contact field on your form. When you select the repeating row for the data source, you can click the Filter button in the field picker. Your filter may vary depending on what the value field is for your first drop down, but basically, you want to make sure that you select the field your secondary data connection that is equal to the contact field in your main data connection.
Yuck - I'm doing a poor job of explaining. See the attached sample (save it locally, open in design mode to preview). Note that if you creating an IP 2007 Browser form this type of filter is not supported on list boxes.