This works -to an extent- already. Take a look at the email head, that's sent out:

A new submission (form: "Subscription Form")
======================================
Submitted on: April 11, 2007 @ 10:26 pm
Via: /test-custom-cform
By 84.153.92.228 (visitor IP)

This information is also saved in the tracking db (if tracking per DB is enabled). To work properly, this requires WP to use permalinks.

Oliver

As a permament fix, you can try to replace

$page = substr( $_SERVER['REQUEST_URI'], 0, strpos($_SERVER['REQUEST_URI'],'?')-1);

$page = (trim($page)=='')?'/':trim($page);

with

$page = "Post: #".get_the_id();

in cforms.php (line ~351 & ~815).
Let me know if this did the trick.

Here is what I tested for the non-ajax form submission:

This line:

$page .= '('.get_the_id().')';

I added right after line 814 (rel. 3.4), disabled ajax support for the form and it produced the desired output in the email:

 The post context is maintained when sending, hence it only works without ajax.

To make it work with ajax, one would need to add another hidden form field including the posts' ID/Title and deliver it to the backend send mechanism. Quite a bit of work involved here.