ZFS is based on NetApp's WAFL
Not only were the Sun engineers working on ZFS fully aware of NetApp's WAFL filesystem, there's reason to believe they took the fundamental ideas from it and stuck them in ZFS. I suggest you read Hitz's blog entry - especially you, g lane - and the ZFS white paper he links to.
It looks like NetApp are more than willing to point out specific aspects of ZFS that they think infringe on their patents, and they're not just making a big deal about copy-on-write - it's about the specific way ZFS implements copy-on-write and filesystem consistency. To quote the Sun whitepaper:
"The file system that has come closest to our design
principles, other than ZFS itself, is WAFL, the file
system used internally by Network Appliance’s NFS server appliances. WAFL, which stands for Write
Anywhere File Layout, was the first commercial
file system to use the copy-on-write tree of blocks
approach to file system consistency. Both WAFL
and Episode store metadata in files. WAFL also
logs operations at the file system level rather than
the block level."