The best part of using custom presence states is the fact that your message will display on phones just like the normal presence states of "Busy", "Be Right Back", etc.
Not sure what I am talking about? Sometimes a picture helps, so check this out:

Those bottom 4 ‘states’ were added by me.
How to make your own
Download
this and unzip into
C:\Program Files\Microsoft Communicator, assuming you used OC 2007 defaults.
Once extracted, edit C:\Program Files\Microsoft Communicator\OCSPresence.xml with Notepad or
XMLNotepad and change the descriptions to your liking.
Import the registry file C:\Program Files\Microsoft Communicator\OCSPresence.reg and restart Communicator.
Universal limitations
You can only define up to 4 custom entries.
Maximum length of the custom presence description is 64 characters.
XML customState availability can be set to “online”, “busy” and “do-not-disturb”.
My implementation's limitations
You must extract the files into
C:\Program Files\Microsoft Communicator\ unless you want to change the .REG file to use a different location
(can be file://, http://, https://)
You will want to change the “GRR” entry unless you are from Grand Rapids, Michigan.
If you want the custom states set for everyone on the computer system instead of the current user, you need to change the .REG file from HKCU to HKLM.
(Mostly only useful for TS servers)
Update: Looks like I'm not the only one that has been playing around with custom states - check out this post
here.
Update: There is now a VERY cool utility to do the hard work for you. Thanks to the author for leaving a comment about it on this post. You can check out the program
here.