I do a couple of different things with my laptop as far as advocacy go. They're fairly subtle, but seem pretty powerful just the same. I do independent winders support in my local area - mostly your average housewife/computer user.
The first thing I do is use this pic, http://184.108.40.206/spot/getlinux.png, as the background for my gui login screen. That gets some attention right away when a client sees it. Some of them ask what it's all about, and then we get into a discussion about Linux, what it is and what it can do.
Another thing I do is use a transparent terminal (http://220.127.116.11/spot/snapshot7.png). That *really* catches eyes, and quite a few ask how they could get something cool like that on their machine, which leads to yet another conversation about linux, how/if it would be right for them, yada yada yada.
One thing I *never* do with a client - rant about winders' shortcomings. The last thing someone wants to hear is how lame something they might be doing/using is - no one likes to be talked down to. In fact, I rarely even mention "Linux vs. Winders" at all, unless there's something specific that's asked, though there were recently a TON of questions, as everyone one my ISP got the klez worm emailed to them.
See the following articles, they may be of some help:
the second one has a part that might look familiar