VMotion across cluster could be done. As long as EVC is enable and is compatible to eachc others. Example will be Intel Processors and not mix mode with AMD processors in different cluster. To do this, you need to ensure the vmkernel is able to communicate with the rest of the clusters you wanna vmotion to. At the same time, all the datastore should be presented to each of the host to ensure vmotion success.
For Offline migration, you can move it to any host you like with datastore changes too.
Craig

vExpert 2009
Malaysia VMware Communities -
Craig
vExpert 2009 & 2010
Netapp NCIE, NCDA 8.0.1
Malaysia VMware Communities - http://www.malaysiavm.com