Have you got a php script to use Curl to do the posting of the data back to PayPal? I want to work on an automated subscription feature for my site as well but haven't gotten around to it. If you have already done this I would be very interested in it as well.
If not I'll post what ever it is I come up with when I make it. I didn't want to use the cgi script they provide but rather wish to keep everything php.