Here is the text of a reply I received from Nik Software when I asked about compatibility of their plug-ins with CS5:
"Currently Nik Software has been diligently testing the beta version of Photoshop CS5 with our products. While we are aware of a couple issues, described below. Aside for those issues, the Photoshop CS5 programming code is continually changing and for this reason, we cannot say for certain if our products will be fully compatible upon the final release.
"Once the final version of Photoshop CS5 is released, we will then be able to know entirely what issues, if any, may need to be addressed. Nik Software is committed to our Photoshop customer install base and should any updates be required, we will address these as soon as possible. Such updates can take some time to create and get out to our customers.
"In order to ensure a smooth workflow, we recommend waiting to upgrade to Photoshop CS5. Once we have fully evaluated the final release, and have determined if any updates are required, you can then make an informed decision on when to upgrade to Photoshop CS5.
"There are two know issues that we are aware at this time. One relates to Windows 64-bit users, and the other one relates to Macintosh users. These issues are subject to change until the final release; however, we do not expect these issue to change. Windows 32-bit users are not affected by these issues.
"For Windows 64-bit Users (XP, Vista, 7):
Photoshop CS5 for Windows will continue run in both 32-bit and 64-bit version. Those using a Windows 64-bit machines will need to continue to use the 32-bit version of Photoshop CS5 (the same as was the case with Photoshop CS4) for all current products, except for Viveza 2, which is fully compatible with both 32 and 64-bit versions of Photoshop.
"For Mac Users (10.5 Leopard and 10.6 Snow Leopard):
Photoshop CS5 will for the first time, run in 64-bit on Mac OS 10.5 (Leopard) and Mac OS 10.6 (Snow Leopard). Currently all Nik Software products are 32-bit compatible. For this reason, none of our products will work natively in Photoshop CS5 while in the default 64-bit mode. To work around this issue, until the 64-bit version of Nik Software products are released, Photoshop CS5 will need to be forced into 32-bit mode.
"Typically the user would need to locate the Photoshop CS5 application file (Macintosh HD | Applications | Photoshop CS5. Once located, select it, press Command + I (apple key + I) to get the Information dialog. In the information dialog window, midway down there should be a check box that will allow you to select "Run in 32-bit Mode", then close the information window. Once this is done, each time Photoshop CS5 is launched it will automatically be running in 32-bit mode and all Nik Software product will work as they did in previous versions of Photoshop.
"Nik Software is currently evaluating 64-bit compatibility; however, at this time we do not have any information on when our products will become fully 64-bit compatible."