It was quite brilliant really. I just ran the following command and followed the prompts:
So far, everything looks good. New kernel (2.6.32 from 2.6.24), MySQL (5.1 from 5.0), Apache, Postfix, slapd, etc etc. The one that looks like needs some babysitting is Dovecot which requires an updated config file.
Everything else worked "out of the box". And I'd consider this system fairly customized in the sense that a wide variety of applications have been installed but where possible (and almost entirely), taken from the Ubuntu repositories.
So if there's anyone else out there still waffling, do it! Do the upgrade!