While browsing ThinkGeek.com, I came across a caffeinated soap called “Shower Shock”. They claim that the caffeine is absorbed through the skin as you wash. Is this possible?
Although I highly doubt that you can absorb caffeine though the skin this way, I now own a bar for the sole purpose of freaking out house guests. 
You can view the product at http://www.thinkgeek.com/stuff/caffeine/5a65.shtml .
Apparently, there is some validity in the idea that caffeine can be absorbed through the skin:
http://www.pharma.tno.nl/Product.cfm?PShID=650&DivID=9
You will note, however, that the sample study described there involves a 4 hour application. I have my doubts that you can get anything but a boost to your imagination from caffeine infused soap which you wash off again immediately, and probably doesn’t contain that high a concentration to begin with.
The soap also contains peppermint oil, which may produce a noticeable effect, that one might interpret as “invigorating” if one wished.