So far, I have customized shortcuts to toggle the file browser on and off (F3) and to open a new Window (Shift-Ctrl-W). These command don't have default shortcuts, so I created my own. I also swapped the default shortcuts for the Open and Browse commands because I would prefer to use Ctrl-O to open the File Browser.
Before you start customizing, I suggest you first generate a summary of the default key assignments. To do that, go to Edit > Keyboard Shortcuts and click the Summarize button to create a chart of the default shortcuts. You'll be prompted for a file name and after you save the file it will be opened in your Web browser. You probably don't need to print that file if you plan to customize your own shortcuts, but it could be useful to refer to later.
Shortcuts can be customized for all the menu commands, the palette menus, and the tools. Switch between them using the "Shortcuts for" menu. For application and palette menus you will need to click the triangle to expand the list for each menu before you can see the list of commands. To change one, click on it and a box will appear with a blinking cursor. Press the keys you want to assign as the shortcut and Photoshop will alert you if there are any problems.
You can assign more than one shortcut to the same command by clicking the "Add Shortcut" button. If you choose a set of keys that conflicts with another shortcut already assigned, Photoshop will let you know so you can either undo the change, or accept the change and go directly to the conflicting command to re-assign it.
If you choose an F-key shortcut that is already assigned to an action, it will allow you to accept the shortcut, but the action will override the command until you manually go into the action and remove the shortcut assigned to it.
For instance, I wanted to use the F3 key to toggle the File Browser on and off. When I changed this, Photoshop warned me that the F3 key was already in use by the action titled "Copy (selection)." I clicked accept, but the F3 key would still perform the Cut action instead of turning the File Browser on and off. To change it, I had to go into the actions palette and find the specific action with this key assigned, then double click the name to bring up the action options. From there I could remove the F-key assignment or add a modifier so that it would not conflict with my Browser shortcut.
Once you've customized the shortcuts to your liking, you should save your set and generate a new summary. Click the disk button next to the set menu to save the set permanently, otherwise the changes will be lost if you ever need to trash your preferences. Also, when you save the shortcuts you can return to them at any time--useful if you use Photoshop on more than one computer or a computer that is shared with other users. Just copy the *.kys file from the Photoshop CS\Presets\Keyboard Shortcuts folder and take it with you! If you've customized many shortcuts, you will probably find it useful to click the summarize button and print a list to keep nearby until you learn them all.
As much as I like this new ability to customize the keyboard shortcuts in Photoshop CS, I still miss the visual Quick Reference card that came with previous versions of Photoshop.