Well, PHP5 doesn't seem to make any difference.

Which version of Ãœbercart are you using with your updated uc_simpleinventory? And (perhaps a stupid question) I assume you have verified that your stock levels are actually decreasing in the uc_simpleinventory table when ordering? I assume the decrease is supposed to occur when you click the last "submit" button...right before being taken to PayPal?

P.S. I'm currently trying the bazaar version of Ãœbercart.