First try on upgrading to the new 5.1 appliance and a big fail with a filled destination 5.1 vCSA root partition. (( Upgrade that worked here ))
If you have configured remote syslog on your source appliance, this routes to a /new directory on the destination appliance during the upgrade (not /storage/log). Unlucky for us, VMware choose not to have this /new directory with it's own mount point.
My work around is to create a new 20G drive and mount to /new before starting.
Oh, and take a snapshot BEFORE importing/swapping keys between source and destination vCSAs..
I'll update if I come across any other gotchas.
I've pinged VMware's kb with this, hopefully they'll get something up quickly. As for me...to last night's backup I go. Good thing this isn't production.