Peak files - ones with .pk and .pkf extensions - are files that Audition creates upon first opening an audio file, and they contain information about the way it's displayed on the screen. After you've worked on a file, saving the peak data as well as the file means that the next time Audition opens it, it can do so a lot faster - simply because it doesn't have to create all that visual data again.
By default, Audition saves peak files automatically for wav files, but it can't create them for other types. The reason for this is that it doesn't need them for those file types, as they can't be opened natively in Audition anyway; they have to be decoded into wav files first.
The difference between older .pk files and newer .pkf files (the ones that Audition CS5.5 uses) is that the .pkf ones contain Floating Point data. This means that unlike .pk files, they can display information about files where the signal exceeds 0dB. This means that if you do a mixdown where the levels are rather hot, you can display the levels correctly in Edit View by zooming the vertical display out. If you replay the file at this level though, it will distort, so you still have to normalize your resulting file so that the peaks don't exceed 0dB before making, say, a CD from it, a 16-bit integer copy, or any sort of compressed file.
Because of this fundamental difference though, .pk files and .pkf files aren't interchangable. So a .pkf file created with Audition CS5.5 won't work with any previous version. What will happen if you open one of these files in, say Audition 3 is that a new .pk file will be created, and stored alongside the .pkf one. And similarly, a .pk file from a previous version will be ignored if an older file is opened in Audition CS5.5 - it will create a new .pkf file for it.
Both .pk and .pkf files are normally hidden file types, so you won't see them in your files folder unless you've set it to display hidden files. It doesn't matter in the slightest if one gets deleted accidentally - which ever version of Audition you are using will recreate the appropriate one automatically. The only difference you will notice is that it takes longer to open the file. Not much of a problem with a very short one, but if your file is an hour long, the difference is significant.
In the light of a few recent posts about this, I should also add that you shouldn't try to open .pk or .pkf files on their own. By default, the file extensions aren't associated with Audition as far as the OS is concerned - they don't need to be, because Audition will look for the peak file associated with a wav file when it opens it, and the OS doesn't get involved in the organisation of this. And, on top of that, Audition doesn't regard the mere existence of a .pk or .pkf file as an excuse for opening. Why should it? It knows that there's no audio in it!
So basically - You Can't Open .pk or .pkf Files.