<update 21-Jan-2013>I have just been informed that this issue was fixed in vSphere 5.0 Update 1. The KB article and 5.0 U1 release notes will be updated shortly!</update>
If you are using a VAAI ATS capable array and VMFS-5 you might want to read this KB Article. The article describes a situation where it is impossible to mount VMFS volumes when they were formatted with VMFS-5 on a VAAI ATS (locking offloading) capable array. These are the kind of problems that you won’t hit on a daily basis but when you do you will be scratching your head for a while. Note that this also applies to scenarios where for instance SRM is used. The error to look for in your vmkernel log is:
Failed to reserve volume
So anyone with a 5.0 environment and newly formatted VMFS-5 volumes might want to test this. Although the article states that so far it has only been encountered with EMC Clariion, NS and VNX storage, it also notes that it might not be restricted to it. The solution fortunately is fairly simple, just disable VAAI ATS for now.
esxcli system settings advanced set -i 0 -o /VMFS3/HardwareAcceleratedLocking
For more details read the KB and I would also suggest following it with an RSS reader if you have this issue, that way you get notified when there is an update.