Tomorrow, all being well, I'll receive a new computer.
I've always run Debian unstable upon my desktop in the past, partly because I wanted to have "new stuff" and partly because I needed a Debian unstable system for building Debian packages with.
However I'm strongly tempted to just install Lenny. I use that upon my work desktop and it does me just fine for surfing, building tools, and similar.
I can use pbuilder, sbuildd, or similar to build packages for upload to Debian, and if I want to experiment with new-hotness I can use a KVM guest or two.
Providing the hardware works with Lenny (and I have no reason to believe it won't) then there's no obvious downside I can think of.
The only potential complication will be restoring my backups, it is possible that my firefox databases, and similar things, might not work on older version. Still we shall see.
I plan to install software RAID, and run the system on LVM because quite frankly it rocks. Unless my current system fails in the next 24 hours I can use that to do the installation (My current desktop acts as a TFTP/DHCP/NFS server so I can use it to PXE-boot).
Anyway now I need to go eat food, tidy my desk, and decide what to call the machine .. At the moment the choice is between "march.my.flat" and birthday.my.flat, as my 34th birthday is on March 10th.