I was running a Xen server with Debian Etch as dom0 (Linux 2.6.18-6 with Xen 3.0.3-1 on AMD64) for some time now. Today, I decided to upgrade the dom0 to Debian Lenny (Linux 2.6.26-2 with Xen 3.2.1-2). More here
The domUs are all running a Debian-based OS (3x Lenny, 1x Ubuntu Hardy). The upgrade was quite straightforward, however there were some pitfalls you can avoid in advance.