Ah, I just had a look on Wikipedia
(http://en.wikipedia.org/wiki/Universal_Disk_Format#Native...), and
I think the reason I could write under Windows XP was because Dell had
installed SonicDLA, which adds write-support to Windows XP machines.
MacOS 10.5 and Windows Vista both have write-support, which is encouraging,
but with Windows XP read-only by default, UDF won't be a viable alternative
for another two or three years.