When it comes time to get out of the office for a while , whether for business or pleasure, it's always a good idea to let those around you know when you're gone, when you'll return and how to reach you while you're away. There are two ways to do this... the traditional (boring) way of using a Post-It note, or the high-tech way... using, you know, technology.
When it comes to leaving a high-tech note, there are two ways that I recommend doing this.
Keynote (or PowerPoint)
If you want to leave a note using Keynote or PowerPoint, it's simple. Design a few slides with the details of your absence, then set your slideshow to loop. When you're ready, just play your slideshow on a computer that will be in plain site while you're away, and let it loop over and over and over again.
Final Cut Pro
If you want to get really fancy shmancy, use a video editing tool like Final Cut Pro to create a short information video. When it's ready, export the video into a format that QuickTime can read (.MOV, .MPG4, etc), then play the video. Be sure to set QuickTime to loop the video (View | Loop}. Here's a sample of what this could look like.
I guarantee this will catch everyone's attention in the office, and it will be impossible to miss. No more "I didn't know you were gone" comments.