"Hi Mike, I was able to clone 10.8 from my MacBook Pro to a spare SATA hard drive in my Mac Pro 1,1. I have been booting the Mac Pro in 64-bit mode using the Chameleon -r1921 boot loader for several weeks, anticipating the release of Mountain Lion. After editing the supported machines plist and tweaking the installed extensions a little 10.8 seems to be running just fine.
   (Not sure if it works for 10.8 (i.e. if it requires the file present), but for OS X 10.7 installs most older Mini owners w/C2D upgrades just deleted the supported models plist, although some did edit it for their model ID. (Either way, sometimes an OS update (combo) may restore/rewrite the plist file so keep a backup if you've edited one.)-Mike)
I didn't try deleting the .plist, I just edited it and I did save a copy in my documents folder in case future updates rewrite it. Most of my help on this came from the two pages you referenced (repeated below), they are excellent resources.

I have a lot invested in this Mac- 2 SSDs, 16 GB RAM, ATI 6870- so I wasn't happy that Apple dropped support of the 1,1 but "unsupported" just means it takes a little extra work.

(he later wrote)
I ran a couple of tests late last night, Rember shows all 16 GB of my RAM working properly (some 1,1 owners have reported RAM problems going to 64-bit) and Geekbench scored 6974, up from 6180 in Lion 32-bit with the same hardware.

(8/25/2012 Update/Note on OS X 10.8.1 update OK)
I sent a note a few weeks ago about running Mountain Lion on a MacPro 1,1 (booting 64-bit via Chameleon).
I updated to 10.8.1 with no trouble, I didn't need to redo the edit on the supported machines plist.
-Dave"

For anyone that missed them earlier, here's links to tips on How I installed Mac OS 10.8 on my MacPro 1,1 and an older thread from 2011 at netkas on Booting the 64-bit kernel on 2006/2007 Mac Pros.

  

• About Drive Cloning with OS X 10.7 or Later (sponsor blog)
• Safari 6 crashing in Mountain Lion
• (Forbes) Mountain Lion Drops Battery Runtime Like Lion Did
  (ref: Apple MBP forum thread Battery life dropped considerably on Mountain Lion)
• Apple A4/A5 Designer & K8 Lead Architect, Jim Keller, Returns to AMD
• Apple designer describes company's 'kitchen table' in testimony
• Hulu blocks overseas Apple TV owners streaming via US iTunes accounts
• Amazon Takes on Netflix, Hulu With Instant Video iPad App
• Amazon Instant Video for iPad app (itunes app link)

• OS X Mountain Lion: Font list
• OS X: How to change the language displayed in the login window
• About diagnostic and usage data in OS X Lion and Mountain Lion
• Installing QuickTime Player 7 (in OS X 10.6, 10.7, 10.8)
• OS X Server (10.8): Shared folders on external storage no longer available after logging out locally from the server (includes tip/fix)
  
  
• iCloud: Keeping an existing @mac.com email address
• iCloud: What do Back to My Mac status messages mean?
• iCloud: Distinguishing between Calendars and Reminders
• iCloud: Manually importing data from Calendar or iCal to iCloud Calendar
• iCloud: What to do with your iCloud data before selling or giving up your Mac
• iCloud document icon not displaying correctly after temporary data cleaned out of app memory
  
  
• iMovie: MobileMe services are no longer available
• MobileMe iDisk: About previous local iDisk files
• How to move an iWeb site previously published to MobileMe to another web hosting service
  
  
• Find My Friends: Frequently asked questions
• Find My iPhone: Changes after moving from MobileMe to iCloud
• iPhone and iPod touch: Search with Mail accounts and servers
• iOS: Unable to use webpages or services that require cookies
• Recovering iCloud or MobileMe data from iTunes backups for an iOS device
  
  
• Aperture and iPhoto: How to sign in to place an order
• iPhoto 9.2.2 or later: Migrating MobileMe albums and photos to your iPhoto library
• Aperture 3.2.3 or later: Migrating MobileMe albums and photos to your Aperture library

• The Intel SSD 330 Review (60GB, 120GB, 180GB) (anandtech)
• Amazon tops iTunes Match offer with more storage
• Should You Upgrade or Replace Your Laptop?
• Google delays Nexus Q, offers freebies in return
• Email-Based Malware Attacks, July 2012
• Dropbox: (Employee) Password Breach Led to Spam
  (ref: yesterday's Dropbox blog post on Security update and new features)
• Warning: Invitation Facebook Virus e-mail is a hoax (again)
• Amendment to 2012 Cybersecurity Act attempts to give Federal datacenter consolidation mandate some teeth
• Verizon to Allow 3rd-Party Tethering Apps on Android Phones
• Review: Sony Cyber-shot DSC-RX100 (Small camera w/large sensor)
• Oculus Rift virtual reality headset gets Kickstarter cash