Finally got around to setting it up. Was pretty painless (worst part was trying to find a place to store my data and then get it back onto the fusion drive).
I'm using a 240GB SanDisk SSD and a 320GB 7200 RPM HD in my MacBook Pro. Shows up as a single 556GB drive.
dynamic=:~ Td$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *240.1 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_CoreStorage 239.7 GB disk0s2
3: Apple_Boot Boot OS X 134.2 MB disk0s3
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *320.1 GB disk1
1: EFI 209.7 MB disk1s1
2: Apple_CoreStorage 319.2 GB disk1s2
3: Apple_Boot Boot OS X 650.0 MB disk1s3
/dev/disk2
#: TYPE NAME SIZE IDENTIFIER
0: Apple_HFS Fusion *556.1 GB disk2
At the actual partition level, there's the usual EFI partition on both disks, then the CoreStorage partition that's used to create the Fusion disk, followed by an Apple_Boot partition. On the HD, this seems to act as the Restore partition for Mountain Lion.
If you boot to Lion, you'll only see a single Boot OS X partition mounted (I can't remember if it was the SSD or HD one). All of your data will be hidden.
I only tested speeds briefly, but they were noticeably worse than the SSD alone. ~130MB write vs ~250 and ~350MB read vs ~500MB. I'm not sure if TRIM on my drive is just slow or what, so I'll try again once it's had some time to settle. My results seem to be opposite of what other reviews have shown.
One thing that I'm still looking into is if the HD will spin down on it's own when not in use. That's the biggest annoyance I had with a 2 disk setup on my MBP - the HD would spin up/down all the time unless I specifically unmounted it. So far it seems to stay spinning all the time :(