Just an addendum from yesterday's post on Git at Google Code – the wiki pages are backed by a version controlled repository. As a result, if you migrate from one repository format to another, your wiki pages will need to be migrated as well.
The locations are shown on the wiki support page:
If Subversion is used, all wiki pages are stored in the /wiki/ directory of your project's Subversion repository, as files ending in .wiki (other files and subdirectories will be ignored). That means you can use a Subversion client and your favorite text editor to add and edit wiki pages.
If Mercurial is used,the wiki content is stored http://wiki.[projectname].googlecode.com/hg/ where [projectname] is the name of the specific project. For Git, http://wiki.[projectname].googlecode.com/git.
If you have a Google Code project primarily for issue and wiki support, and host your Git content elsewhere, then you need to ensure a seamless transition of your wiki pages to the new Git repository after conversion, as otherwise it appears that all your wiki pages are just deleted.