What do programs see when ZFS can't deliver uncorrupted data?

EIO is indeed the only answer with current ZFS implementations.

An open ZFS "bug" asks for some way to read corrupted data:http://bugs.opensolaris.org/bugdatabase/printableBug.do?bug_id=6186106

I believe this is already doable using the undocumented but open source zdb utility.Have a look at http://www.cuddletech.com/blog/pivot/entry.php?id=980 for explanations about how to dump a file content using zdb -R option and "r" flag.

Solaris 10:

# Create a test pool[root@tesalia z]# cd /tmp[root@tesalia tmp]# mkfile 100M zz[root@tesalia tmp]# zpool create prueba /tmp/zz# Fill the pool[root@tesalia /]# dd if=/dev/zero of=/prueba/dummy_filedd: writing to `/prueba/dummy_file': No space left on device129537+0 records in129536+0 records out66322432 bytes (66 MB) copied, 1.6093 s, 41.2 MB/s# Umount the pool[root@tesalia /]# zpool export prueba# Corrupt the pool on purpose[root@tesalia /]# dd if=/dev/urandom of=/tmp/zz seek=100000 count=1 conv=notrunc1+0 records in1+0 records out512 bytes (512 B) copied, 0.0715209 s, 7.2 kB/s# Mount the pool againzpool import -d /tmp prueba# Try to read the corrupted data[root@tesalia tmp]# md5sum /prueba/dummy_file md5sum: /prueba/dummy_file: I/O error# Read the manual[root@tesalia tmp]# man -s2 read[...]RETURN VALUES     Upon successful completion,  read()  and  readv()  return  a     non-negative integer indicating the number of bytes actually     read. Otherwise, the functions return -1 and  set  errno  to     indicate the error.ERRORS     The read(), readv(), and pread() functions will fail if:[...]     EIO        A physical I/O error has occurred, [...]

You must export/import the test pool because, if not, the direct overwrite (pool corruption) will be missed since the file will still be cached in OS memory.

And no, currently ZFS will refuse to give you corrupted data. As it should.

How would returning anything but an EIO error from read() make sense outside a file system specific low level data rescue utility?

The low level data rescue utility would need to use an OS and FS specific API other than open/read/write/close to to access the file. The semantics it would need are fundamentally different from reading normal files, so it would need a specialized API.