I've got a non-critical web server that I'm thinking of upgrading to Debian Squeeze now that it's been frozen for a while. Are there any Linode specific reasons why I shouldn't do this? I've been running debian testing or unstable on my desktop for over five years so I know what's involved with that side of things.

No Linode specific reason that I can think of. Note that the udev package in squeeze needs a recent kernel and vice versa. But that's not Linode-specific either I guess.

I assume that you have ruled out using lenny + lenny-backports...