The zone could also be hosted on an IP in a segment of addresses that is temporarily unavailable to certain regions (BT had issues with this in the last year because of problems with the BE/O2 segments crapping out).

Please, when reporting these issues, complete the bug report info as best as you can--specifically paying attention to your connection details (Cable/DSL/Dial up, ISP, and your region will go a long way towards tracking things down).