In the debian/rules file. It is basically just a Makefile so look for the debian/contorl rule. However, that "/usr/src/wxwidgets/wxPython-src-2.8.1.1" shouldn't be there at all, so look further up the file for where the value of sorelease is set.
Wait, I think I remember seeing something like this before. There is something screwy in there that uses the name of the source directory to set one of the values that goes into the package names. Since the default doesn't match the expected pattern it is causing this build bug. Try changing the name of the dir from "wxPython-src-2.8.1.1" to "wxwidgets2.8-2.8.1.1".
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Robin Dunn
Software Craftsman http://wxPython.org Java give you jitters? Relax with wxPython!