I did not upgrade to Joomla 2.5, it was to Joomla 1.7 - as you said there is not much point moving on until the extensions I use (CB) are ready for it.
Also yes it was the 1.7.1 release of CB, I have performed a number of tests and so far the steps I described for upgrading seem to have been successful. This site did not have any other CB extensions apart from FB Connect and as pointed out this did require further work but nothing a bit of SQL manipulation couldn't fix.
The good news is that JUpgrade does keep all of the user IDs in tact and as such the rest of the migration is a cake walk.
Thanks for your feedback on this. I think I can rest easy