As previously mentioned, this is not 10.6 specific. Only GUID is supported for Intel machines and the Install discs will not allow an install to an APM drive in an Intel machine.

One early official source:
http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh2315.html

As part of the "Mac OS X Support Essentials" course (book available through Peachpit Press) it is thoroughly covered (and part of the test) that from an official support standpoint, PPC will only boot APM and Intel will only boot Intel.

The reality, however, is that an Intel machine will boot either. PPCs will only boot APM, though.

So to have a "diagnostic drive" that contains boot and/or installers for either platform, you can/should use APM.

In order to get around the technical issue mentioned, my solution is to:

1) Partition an Intel machine as GUID
2) Install Mac OS X
3) Boot off an external drive that contains Disk Utility
4) Make a file-level (New Disk Image from Folder) clone of the drive
5) Partition your "diagnostic drive" as APM and include a space for your new OS
6) Restore the disk image to your drive

Using this method, I have an APM drive (triple-interface) with the following:

* Boot 10.4 PPC
* Boot 10.6 Intel
* Install 10.4 PPC
* Install 10.4 Intel
* Install 10.4 Server
* Install 10.5 Client (Universal)
* Install 10.5 Server (Universal)
* Install 10.6 Client (Intel)
* Install 10.6 Server (Intel)
* Lastly, a large Tools/Cargo partition for common apps, etc.

I've been using this method since the oldest installer on my drive was 10.2 and the newest OS was 10.4 (Intel), so I'm very confident it will work for you.

Good luck!