xISBN was down for nearly 24 hours, which gave lots of people problems, including several of us here at OCLC. It was an upgrade which ran out of space and then went from bad to worse.
The good news is that the updated data is from a just-pulled copy of WorldCat and the software has been extended to handle 13 digit ISBNs, or at least 13 digit ISBNs which start with 978, which I believe all the valid ones currently do. We went back and forth on how to accommodate the longer numbers and ended up with a fairly simple solution that shouldn't break anyone's code. If we receive a 13-digit ISBN we silently convert it to the equivalent 10-digit one and proceed from there.
A side effect of doing it this way is that xISBN functions as a 13-digit to 10-digit ISBN converter, since we always echo the sent ISBN even if we don't find it in our table.
We're sorry we were down and hope that what we've done with 13-digit numbers seems reasonable. Someday real-soon-now OCLC is going to make xISBN a supported service, which should be more reliable (I'd guess our reliability is better than 99%, but not the 99.5% or better such a service really should be).