If you want to block anyone just from seeing you online on Facebook IM or Facebook Chat, follow these steps (Accurate as of 6/3/2010):

1.  With your Facebook page open (and logged in).  Click on Account |  Edit Friends.   (Upper right hand corner of the screen)

2.  Click the button that says “Create New List”

3.  A box will open that says “Create New List” and asks you to enter a name.  Type “Blocked IM Friends”.  (You can put anything you want…you’re the only one who will see this name)

4.  Under that box you can type the person’s name or you can select them in the list below.  You can add multiple people.

5.  When you’ve selected the Friends you want to block, click on the button that says “Create List”.

6.  Now that you’ve created a list and added people to it, you need to block that list on Facebook chat.  Do this by clicking the Chat button in the lower right hand corner of your Facebook page.  You should see the name of the List you just created with a green oval  next to it.  Click on that green oval (it will turn gray) and everyone in that group will stop being able to when you are online for IM.  You can turn it back on at any time by clicking the oval again.

You can create other groups like this as well.  I have groups for co-workers, high school friends, etc.  You can also use these same lists when you post just about anything to include or specifically exclude friends (or groups of friends) from seeing that post.