You might have heard that VMware were bringing out the ability to migrate ESX VMs between storage areas (e.g. to different SAN LUNs or local disks) whilst running. Good news is that it part of ESX 3.5: http://www.vmware.com/products/vi/storage_vmotion_overview.html
Bad news is that is that Vmware haven't integrated the functionallity into their Virtual Infrastructure Client, however a clever person at LostCreations has created a plugin: http://www.lostcreations.com/code/wiki/vmware/viplugins/svmotion
To use this fantasic tool, you'll need servers running ESX 3.5 and to install the LostCreations SVMotion plugin for Virtual Infrastructure Client following the installation instructions on: http://www.lostcreations.com/code/wiki/vmware/viplugins/svmotion
(you don't need to install the VMware Remote CLI which is required for "manual" storage vmotion)
Have tested and works a treat with no interruption to the VM during migration.