The Ubuntu "popcon" package is installed when a user opts to share anonymous usage data during the install process (or within the synaptic package manager).
Analyzing the data (http://popcon.ubuntu.com/), we can see the following for Wine:
wine 215262 installed 41731 use regularly 156710 don't use
compare this with dpkg, which every Ubuntu system has:
dpkg 576989 installed 151098 use regularly 411213 don't use
Doing some division, this means that a rather high 37% of Ubuntu users have Wine installed, and 10% of all Ubuntu users actually make use of Wine. 7% use Wine "frequently", however popcon defines that.
Combine this with other estimates of Ubuntu's user base (about 8 million last I heard), and you have approximately 800,000 Wine users.
This data is slightly biased - it doesn't include most Wubi users who installed Ubuntu from within Windows, since they aren't given any installation questions. I believe, though can't prove, that this means we are under-representing Wine users, as it seems reasonable that Wubi installs would be more likely to have Windows applications to run and thus need Wine.
Thanks, Scott Ritchie