One of the e-comm packages I work with has an option to use WorldPay in conjunction with the cart. You need to have a separate WP agreement, but once they send you the account details, you enter that into the e-comm package, and it wouldn't validate a txn until it got the authorisation code back.
Then WP changed the formatting of the data packets they sent and recieved, without telling anyone, and that of course broke every e-comm site we had that was using WP. Fortunately, that wasn't too many, and it got fixed in a hurry, but still...
From the timing of spikys post, it seems he got stung by more or less the same thing, the timing seems to fit. I seem to recall that they also changed the structure of their customer accounts at the same time.
Again, fortunately for us, WP headquarters is just down the road from the company I was working for at the time, and one of our directors was friendly with WP, so we got sorted out in a hurry. Not everyone was so lucky it seems