Your original post said a client postback, not a client callback, hence my confusion.  By default, a callback will only contain the arguments passed in it.  However, as the Client Callback document states

Accessing FORM Values In A Callback

Starting with version 1.2 of the ClientAPI (included in DNN 3.3/4.1), you will now be able to specify that your callback should contain the values of one or more FORM elements.  For example, if you wish to have the entire up-to-date form values pushed to the server you would register your client callback script like this.

Note:  The last parameter (Form) specifies the element ID whose children we wish to post the values for.

You would then be able to access the form values during a callback through code like this.

Note:  You cannot use the Me.txt.Value property since a callback will be processed before the control values are populated

You should note that you only want to post back the values necessary for your callback in order to keep your payload as small as possible.  The Register function allows you to pass in any client-side id and it will loop its child controls and only post those values.  For example, you could have a DIV tag containing 3 textbox controls, by specifying the DIV tag’s client-side ID as the last parameter, those three textbox control values will be accessible in the callback.

You may recall, that the way the ClientAPI passes variables back and forth through its setVar/getVar functionality is with a HIDDEN FORM element.  Due to this, it is easy to only post this information for your callback.

Note:  A constant has been defined within the ClientAPI namespace that represents this hidden variable (__dnnVariable).

If you have registered your callback script specifying the dnnVariable or the entire form you will be able to access your variables through the normal method

Note: You will not be able to set any values and have them reflected on the client.