How do you pass an ID to a sControl?

If the info being passed in can be derived solely from merge fields (in this case perhaps it can be) and you want the "subject" of the target scontrol to be the source object. If you really only want the source contact's ID and  wish to have access to merge fields for an entirely different target object + some arbitrary data from the target you can accomplish that today using a combo of s-controls and the ajax toolkit.

Now the good news: this is a highly requested feature that has been implemented in the next major salesforce release (feature is already baked in). Basically, you will be able to get the value of query params /or form fields using the new $Request object, e.g.:

var id = '{! $Request.myQueryParam }';
alert('You passed in ' + id);