Without encryption, compression, or deduplication on I don't think there would be very much overheard - more than traditional file systems, definitely, but still relatively very little.
Deduplication requires lots of memory (I think I saw 1-2GB per TB of data quoted someplace) and will increase CPU load a bit, but for certain workloads can save a lot of writing and diskspace (think Virtual Images or a dataset like Dropbox's).
I'm not involved in ZFS development (just an avid user) so take this with a grain of salt.
With new Core iX processers now being able to do AES in hardware, I wonder if the CPU requirement for encryption will go down significantly once/if support is enabled.
ZFS lets you put encryption/compression/dedupe on a per filesystem basis, and filesystems are extremely light and easy to create in ZFS (one/multiple per home directory or installed app is normal) so it's easy enough to only apply those features where they're needed.
As ZFS checksums every block in the CPU anyway, doing an additional crypto step isn't all that bad, especially in a server environment where there cores are plentiful.
ZFS will also verify the checksum of each data block, which means that by default it is not a zero-copy filesystem. sendfile(), recvfile(), etc. will all require at least one read pass.
This isn't necessarily a bad thing--in my experience, ZFS has caught on-disk bit rot several times. However, it may not be worth it on an iPhone or other low-power device. Fortunately, it can be disabled.
Encryption would require significantly more CPU (http://forums.freebsd.org/showthread.php?t=9886)
Compression would require moderately more CPU (depends heavily on the compression algorithm) though you can actually speed up disk access with this - which is the real bottleneck (http://blogs.sun.com/observatory/entry/zfs_compression_a_win...)
Deduplication requires lots of memory (I think I saw 1-2GB per TB of data quoted someplace) and will increase CPU load a bit, but for certain workloads can save a lot of writing and diskspace (think Virtual Images or a dataset like Dropbox's).
I'm not involved in ZFS development (just an avid user) so take this with a grain of salt.