Problem handling Paypal IPN + PDT

[eluser]captainredmuff[/eluser]
I'd be interested to see a response to this one as i am having a similar problem with PayPal IPN. Everything works fine in the sandbox but as soon as it goes live, i get an INVALID response even though money has changed hands. Obviously this isn't a viable option for a live site but i'm at my whits end and don't want to start writing hacks to work around this issue.