Piggybacking on my last post where I highlighted Rick Vanover’s Top 5 Tips for Migrating to ESXi, here are some additional tips to help in your ESXi migration efforts. 

Ensure your hardware is supported
Remember, starting with vSphere 4.1 only 64bit hardware is supported.  This also applies to vCenter Server, which in addition to a 64-bit server also requires a 64-bit Windows OS.  Take the time to verify your hardware is supported by checking the vSphere Hardware Compatibility Guide.

Take advantage of the 60-day vCenter trial
Tools like vMotion, Storage vMotion, and Host Profiles come in handy when migrating as they make it easy to move VMs around and reconfigure hosts.  However, not everyone has these features licensed.  Fortunately, when you install/upgrade vCenter 4.1 you get a free 60-day trial that includes all the vCenter features.  Take advantage of this 60-day trial period to leverage these advanced vCenter features to facilitate your ESXi migration. 

Pay attention to VMs and templates on local datastores
Unfortunately, it’s not uncommon to find VMs running on local datastores.  While this is not an ideal place to store a critical VMs, it does happen.  Having VMs on local storage makes evacuating the host a bit more challenging.
Running VMs on the boot disk datastore
Most users will reuse the ESX boot disk when they migrate to ESXi.  In this case it is important to understand that when ESXi is installed the boot disk gets repartitioned and any existing data on the boot disk will be lost.  If you have VMs or templates on the boot disk datastore you need to either migrate them onto shared storage (using Storage vMotion), or perform a reliable backup so they can be restored afer the migration.

Running VMs on local disk datastores
Unlike the boot disk, local disks do not get repartitioned during the ESXi install.  However, the VM registrations will be lost.  Ideally, you should migrate all local VMs onto shared storage, however, if you cannot, or choose not to, be aware that after the ESXi install you will need to manually browse each datastore and re-register each VM.

Use Host Profiles to re-configure your host after installing ESXi
After you install ESXi you will need to manually reconfigure the host for things like vSwitches, Port Groups, NIC teaming, storage policies, iSCSI, NFS, etc.  When migrating one or two hosts this may not be a big deal, but when migrating a large number of hosts this not only become tedious, it is also error prone.  You can simplify the task of reconfiguring your ESXi host with Host Profiles.  After the first host has been migrated and re-configured, create a host profile from the host.  As the additional hosts are migrated they can then be easily re-configured by simply attaching and applying the host profile.  Remember, you have the 60-day trail period during which you can use Host Profiles.

Get comfortable with the vCLI and PowerCLI
There is no Service Console with ESXi.  To perform command line administration you can use either the vSphere Command line Interface (vCLI) or PowerCLI. 

vCLI:  The vCLI is a set of command line tools designed specifically for performing remote vSphere administration from the command line.  The vCLI package can be downloaded from VMware's website and installed on a Windows or Linux VM.  The vCLI is also shipped with the vSphere Management Appliance (vMA) which makes it very easy to deploy and access.

PowerCLI:  On windows systems you can also use VMware's PowerCLI .  Power CLI is also available for download from VMware's website and comes with a rich set of cmdlets that can be used to manage most aspects of vSphere from the command line.

These are only a few suggestions, but I’m confident they will go a long way toward helping you with your ESXi migration.  Good luck!