You will still be able to run 32-bit guest OSes, but the ESX console OS will only work on CPUs capable of Intel VT & EM64T. This is a big deal for two reasons:
1. Dell doesn’t let you enable VT on anything but the PowerEdge 6850, and all ninth-generation servers (x9xx) and beyond. There are a lot of Dell PowerEdge 1850/2850s out there that will still be in use over the next few years. I suspect equipment from other manufacturers, like HP, is similarly affected. This means a lot of people will be stuck at current ESX levels (ESX 3.5.x) for a while until they can get new hardware. That also means that if you want to use some of the new features, like Continuous Availability, you might have to budget for an upgrade.
2. It’s pretty common to recycle old hardware down into test environments, but now you might not be able to do that in the next replacement cycle. If you have, or will have, hardware capable of Intel VT in production make sure you’ve done something to make your VMware test environment VT-capable, too. Even just budgeting now for a VT-capable test server is a good proactive move, to be purchased once the software is actually released.
I have no idea when the next version of ESX will ship, but it’s worth thinking about this sort of stuff now so you can budget for it and have a plan. Even if you decide that the plan is to do nothing and wait.
If you want to test your CPUs to see if they will support 64-bit OSes you can use the VMware CPU Identification Utility. It’s a bootable ISO image that will tell you what you need to know.
To reiterate: ESX console OS going 64-bit, you will still be able to run 32-bit guest OSes, you will need recent hardware to support the 64-bit console OS, and VMware has a tool to tell you where you stand.