Actually I meant the 2.4 branch: I didn't know there was a nestedurls branch. I imagine most of the work there has already been merged into the 2.4 branch. Sapphire (sapphire/) is just part of SilverStripe, along with the CMS (cms/) and so on. All the bits are in different places in the repository; the "phpinstaller" module for whatever branch pulls all the appropriate bits in using SVN's "externals" property. So you just need to pull down phpinstaller/branches/2.4 and give it a go. It's alpha code though, of course.
tobych is correct - The nested urls branch has been merged back into 2.4 and 2.4 is now the maintained branch so you should be using that. The nested urls branch may be deleted soon and projects brought onto 2.4.
It turns out my speed-hump (so far) is actually related to running version 1.4.x of the subversion client: the result was the relative install paths using svn:external references weren't being supported. After trying to fetch a whole bunch of them manually - doing svn propget svn:externals on error paths), I realised I needed to upgrade the subversion client (currently using CentOS 5). Instructions at Stacked Overflow did the job well: http://stackoverflow.com/questions/96597/how-do-i-upgrade-to-subversion-1-5-on-centos-5
So, now happily trying to download the branch, as I'd be keen to try and use as much of the stable codebase as possible with just this little bit of fanciness, rather than going full blown alpha (if I'm understanding the state of this branch correctly - you get mostly stable with the nested URL functionality only as a bonus), and then upgrade to 2.4 down the track.
(realised I didn't hit send, and have since seen Will's comment, and will switch over to 2.4 - hopefully this will still help people with old svn to play nicely).
I don't know how you'd do this; I don't know enough about how the nested URL stuff in 2.4 would work with 2.3. You might be best off contacting Ingo directly or posting on silverstripe-dev, unless someone who knows more happens to read this.