Ok, well I tried it... and I get some odd behaviour. I imagine that it is still to do with the localhost thing. Is that correct?

The order appears to go through from the customer POV, but the order number that is shown is the last successful non-paypal order number... the paypal order never shows up.

Not a big deal... I don't think I like the PP Express thing at all so I won't be using that anyway. And it seems that the IPN is sufficiently unchanged that I can be relatively confident in going live with it (and hope for the best)

- Steven