OK, well I thought I'd report back as to my mind, this is a serious pain in the... Everyone around the web is saying, "Oh just use the relocate command" or glossing over the actual nuts and bolts of doing an SVN upgrade from 2.9.2 to 3.0, but this is all BS. I'm sure there's some very good technical reason why subversion fails when upgrading from 2.9 to 3.0, but it aint very considerate.
Anyway, MrBrian you can't simply delete the hidden .svn directory and checkout the new code, as during the new checkout, Subversion complains that there are already files of the same name in the location you're trying to use. So you end up having to delete all the existing files, in any case.
So, the only way I've found to do is as per Ron R's suggestion, that you have to backup the custom directories (themes, plugins etc) and then do a new install. So deleting all the existing files/directories and doing a new CO with the WP 3.0 code.
Which is quite frankly nerve-wracking enough on a development site, let alone on a clients live site.
I've only just switched to using Subversion in the past couple of WP releases coz I wanted to get away from having to do exactly this type of procdedure (and having a nice fall-back to a previous release if a something goes screwy in the process). Turns out that at least for this release you gotta go back to manually moving/deleting files, and there's no going back once you've gone to 3.0... :-|