I have a XenServer 5.5 in production running Debian Lenny 64bit. It's not officially supported, but it's a known working configuration: Citrix Community article explaining how to set it up.
My question is: are there any problems with update to 5.6? Things to watch for, etc? Or should it just work out-of-the-box?
I have been using Debian Lenny 64 bit on xenserver 5.6 for some time now without any issues. It's worth noting that i didn't migrate a 64 bit machine from 5.5 but setup a new machine.
In my experience a cheap testing environment is a really good thing to have when testing out new releases like your scenario.
The ideal is to have a identical testing environment as your production enviroment but in most cases this isen't a viable option because of the cost.
As a good practice when migrating to newer solutions you should do a isolated test run.
I would setup a isolated xenserver 5.6 with local storage only and create a snapshot from your production debian server that you export to the testing environment.
This way you could try it out without risking downtime on your production system.
It turned out that the XenServer updated more-or-less itself (at least neither I nor any other admin didn't notice prompts about update).
So it looks like it works out-of-the-box.