That's an interesting algorithmic problem. Assuming that you knew the structure of ad-hoc networks in terms of wifi range in a smallish region (for example, by wardriving in a dense urban area), you could pick geographical regions to target for introducing your local "adhoc network meme", which would just involve standing in strategically placed locations broadcasting that ad-hoc network SSID. That would vary with the geographical distribution of WinXP computers in the region. I think the whole social hack could be doable with detailed wifi maps of a region and a laptop. Wonder how much work it would be to implement.