Update

More on x64 here:
/en-us/community/blogs/Lists/Posts/ViewPost.aspx?ID=54

I hate x64. x64 promised to increase the capabilities of computers. More memory, greater speed, bigger, faster, etc… And then, things stop working. Things I like. Things that nobody told me would stop working. Here's a list I started of things that don't work quite right in x64. I'm sorry to say that I discovered most of these the hard way :(

Software that does not run on x64

• Motorola Q9 Update for Windows Mobile 6.1 does not support Windows x64
  The software that updates the Motorola Q9 from Windows Mobile 6.0 to Windows Mobile 6.1 does not run under Windows Vista x64.

• Microsoft Internet Security and Acceleration Server 2006 (ISA 2006) can not install on Windows Server x64
  http://social.technet.microsoft.com/Forums/en-US/Forefrontedgesetup/thread/01e8d66d-8230-4f1b-8fc7-6f648696b94d/

• iTunes does not support Windows x64
  http://docs.info.apple.com/article.html?artnum=301301
  
  Update - iTunes 7.6 supports Vista x64, but there is still no support for Windows XP x64
  
  Update - The following article walks through a [lengthy] procedure for getting iTunes / iPhone to work on Windows XP x64 and Windows Server 2003 x64. While the procedure may work, it does not constitute support. Try contacting Apple if you run into issues. But, it is certainly better than nothing. Thanks for the link, lanceness.
  http://yukichigai.googlepages.com/iphonex64
  
  
• iPhone not supported under 64-bit Windows XP or Vista -- oops
  http://www.engadget.com/2007/06/30/iphone-not-supported-under-64-bit-windows-xp-or-vista-oops/
  
  Update: iPhone is now supported on Vista x64! iTunes 7.6 or higher is required.
  http://www.iphoneatlas.com/2008/01/15/itunes-76-released-windows-vista-64-bit-can-now-sync-iphone/
  
  
• OneCare won't update virus database
  http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2332382&SiteID=2

• Zune Marketplace not supported in x64!, Zune Marketplace software is not supported in XP x64
  http://www.planetamd64.com/index.php?showtopic=28306
  
  Update: Zune Marketplace 2.5 now supports x64! Download it here:
  http://www.microsoft.com/downloads/details.aspx?FamilyId=6136349F-2B32-4946-83B5-A09775531EF4&displaylang=en

• Flash player not supported in 64-bit browser...
  http://forums.firingsquad.com/firingsquad/board/message?board.id=software&message.id=11143

• Microsoft Forefront Client Security does not support x64
  http://technet.microsoft.com/en-us/forefront/clientsecurity/bb404245.aspx

• HP Inkjet Printers - Products Not Supported in Windows XP Professional x64
  http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00364959&lc=en&cc=us&dlc=&product=87608

• Microsoft Fingerprint Reader does not work with x64
  http://www.microsoft.com/hardware/mouseandkeyboard/Download.mspx

• Programs become unstable or do not start after you install an x64-based version of Windows on the same partition as a 32-bit version of Windows
  http://support.microsoft.com/kb/896455
  Have fun setting up that dual-boot environment!

• WebSphere Application Server V6.1.0.11 and later, 32-bit edition, is supported on 64-bit editions of Windows platforms X64 (Intel EM64T / AMD Opteron) hardware. Earlier versions are not.
  http://www-1.ibm.com/support/docview.wss?rs=180&uid=swg21237405
  
  

Software with disabled features on x64

• Send to OneNote 2007 print driver and Internet Fax not available on 64 bit OS. The Office Clean-up wizard not available on 64 bit OS.
  http://office.microsoft.com/en-us/onenote/HA101668651033.aspx

• Visual Studio 2008 does not support "edit and continue" on x64
  http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2277574&SiteID=1

• x64 Compiler Limitations for Windows Driver Kit
  http://msdn2.microsoft.com/en-us/library/ms794341.aspx

• Virtual Server / Virtual PC on Windows Vista
  http://blogs.msdn.com/virtual_pc_guy/archive/2005/08/01/446236.aspx

• Groove File Sharing not supported on 64 bit OS
  http://www.microsoft.com/communities/newsgroups/en-us/default.aspx?pg=2&p=1&tid=ea7444ed-cf8e-426f-af8d-0e1ed146821f&dg=microsoft.public.groove.synch

• Team Foundation Server 2008 application server role will not install on x64 - this means you can't use an x64 server in a single server configuration on an x64 server
  http://internationalized.spaces.live.com/blog/cns!43F3A7682D1564E4!213.entry

• Visual Studio 2008 Extensions Version 1.2 for Windows SharePoint Services 3.0 Tools
  http://www.microsoft.com/DOWNLOADS/details.aspx?FamilyID=7bf65b28-06e2-4e87-9bad-086e32185e68&displaylang=en

• Windows Server 2003 Resource Kit Tools are not supported on x64 platforms. The resource kit tools are not supported? Come on!
  http://www.microsoft.com/downloads/details.aspx?familyid=9D467A69-57FF-4AE7-96EE-B18C4790CFFD&displaylang=en