If you haven't heard that VMware ESX 3.5 U2 inadvertently expires globally today, you probably aren't running VMware.
It appears that VMware mistakenly left a hardcoded timeout in the U2 code. It should have been stripped before the code went out the door, but it wasn't -- so anyone running ESX 3.5 U2 as of today will not be able to migrate or power on any VMs. If they're running, they'll stay running, but if they go down, they won't come back up. There's still no fix available, though VMware now thinks they'll have one ready for this evening, around 6pm PST, hopefully. A media reissue may come tomorrow.
That's a very, very long time.
Fortunately (for me), my VMware farms are safe and sound. That's because regardless of the vendor's track record, unless there's an extremely compelling reason to apply updates to production systems soon after their release, it doesn't happen. This is one of my annoyances with Windows Automatic Updates, and any automated update/patching system. Updating for the sake of updating is very rarely a good thing. Just because the code is newer doesn't mean it's better, and I'd rather that someone else go through the headache of dealing with problems like this. I have ESX 3.5 U2 running in the lab, but I could care less what happens to those VMs. If it ain't broke, don't fix it, indeed.
Of course, not everyone rushed to update and got bitten by this bug -- there are plenty of folks out there putting in their first VMware installations that used ESX 3.5 U2 to start with. What a way to take your first steps into virtualization.
If you're currently dealing with this problem, my sincere condolences.