According to the release notes for the iPhone 2.2.1, SDK XCode now officially supports Subversion 1.5. I've converted my working copy to use the new 1.5 format; however, XCode now will no longer track changes. I'm guessing there is somewhere in XCode that I have to tell it to use the 1.5 format. Has anyone found it yet?
Nothing in the release notes indicates that Xcode comes with svn 1.5, only that it works with it.
I guess that Xcode uses the already installed svn 1.4. Maybe you have to install svn 1.5 on top of it.
I've tried using Xcode's SVN integration and quite frankly, it sucks and I would suggest not using it.
I think you will be much happier with a dedicated Subversion application such as Versions, which is my favourite svn app: www.versionsapp.com
If you don't want to pay for Versions, there is another free alternative called SCPlugin
Oh and to answer your question, unfortunately you are going to have to wait until Apple update Xcode to support the new version, which considering their track record with taking 5 years to give us Java 1.6 on Mac, will not likely be in the near future.
Google is your friend:
http://www.lemonteam.com/blog/2008/12/setting-up-subversion-15-on-xcode/
Please note the errors and caveats in the comments before you go blindly running the code in the tutorial. I have not tried it myself, so this is not a recommendation; merely the best search result I found. Like Brock Woolf I use Versions and the command line, and am quite happy with that workflow.
We have a short tutorial on this that might be helpful:
http://iphonedevelopertips.com/xcode/how-to-fix-subversion-in-xcode-3.html