I would think the only problems would be if you are going to run 64bit OSes, obviously you couldn't run the 64 bit OS VMs on the 32 bit ESX servers.
Also, if you are talking about the same "Farm" it would say its very possible you may not be able to use VMotion between your old and new servers.
Are there any
implications I should know about when adding a 64 bit
box to our Farm that only has 32 bit boxes now?
The most obvious one will be if your doing VMotion and the processor differences, you can perfom masking to help..
This may help..
As a side quesion, does ESX run as a 64 bit OS, or is
it still running in 32 bit mode on the 64bit CPU?
ESX itself i think still runs in 32Bit mode.
This will tell you if your CPU will support 64bit guest VMs.
As a side quesion, does ESX run as a 64 bit OS, or is it still running in 32 bit
mode on the 64bit CPU?
First there is a the Service Console which is RHEL3 based. It's 32bit and AFAIK there are no 64bit plans (what benefit would that give anyway?).
Then there is the vmkernel (the Hypervisor). Since it's all VMware proprietary technical mumbo-jumbo we will never know what they are doing in there. Again, don't care as long as it continues to evolve as hardware assisted virtualization matures.
Are there any implications I should know...
As acr said, there are vMotion requirements/limitations - so choose/design wisely. As hardware assisted virtualisation matures I think we will see this limitation disappear. Fingers crossed sooner rather than later.
As ACR has stated the only phyisical limitation will be Vmotion across hosts this would be a no-no. depending upon how many host you have in your farm it may be better to have two seperate farms one for 32bit hosts and one for 64 bit hosts and buy 2 slightly lower spec server to start the new farm
this is the stituation I have in a small site which has a four node farm 2 x32bit and 2 x 64 bit. ( hardware purchased prior to decision to virtualise)