"daemon" is the term from the UNIX world used to describe programs that run silently in the background, often taking care of important tasks. If you open up Utilities -> Activity Monitor and select to view All Processes, you'll see a host of processes that are running under the "root" user. Most of these are daemons, and are spawned on system startup by Darwin, the BSD UNIX sub-system that OS X runs on top of.
Googling for "pagedaemon" tells me that it is a typical BSD process for managing virtual memory, and pops up whenever a request for virtual memory is made. I could not, however, find anything relating to its use in OS X, or even if it is used at all.
The fact that it is popping up in the dock is strange. My best guess is that you are running a program that may be interacting with the process directly.
Regardless, I don't think it is something you should worry too much about, but at the same time, it really shouldn't be popping up in the dock.
You might stand a better chance of finding a solution by posting in the Mac OS X Technologies forum. Some people there know their UNIX stuff very well.
