Are you doing production work and beta work at the same time? Need to install more than one SDK at once? If you've downloaded the 3.1.3 SDK and need to install it on the same system as the 3.2 beta SDK, you'll want to use the Xcode installer's destination chooser.
When given the option of where to install, click on the Developer folder to reveal the pop-up shown here to the right. To create a new folder/location choose "Other..." and use the standard file browser to create and select that location.
I personally use a named folder for all beta installs. My 3.2 beta is located in /Developer32 and I used the standard /Developer path for my 3.1.3 SDK update. Both SDKs coexist in peace with each other, although I did have to assign separate key macros (I'm a Quickeys addict) for normal Xcode and Interface Builder (in /Developer/Applications) and the beta versions of the same (in /Developer32/Applications for me). Having both SDKs available and separate will help you make sure you can test and code in both environments quickly and easily.