Since the beginning of this year, it is possible to retrieve the number of package downloads of a PPA by using the Launchpad WebService API. You can get more information from bug 139855 and Launchpad API documentation .
I have quickly hacked a program which displays the number of downloads of a PPA or generates a graph.
It requires the name of the PPA and the name of the owner:
ppastats jfi pidgin
Ubuntu Natty amd64 pidgin-libnotify 0.14-4ubuntu10 1589
Ubuntu Natty i386 pidgin-libnotify 0.14-4ubuntu10 2356
Or to generate an HTML page with the graph:
ppastats jfi pidgin --html

The source code can be retrieved from SVN:
svn co http://wpitchoune.net/svnpub/ppastats
The compilation requires the curl and json0 libraries, it can be installed using:
sudo apt-get install libcurl4-openssl-dev libjson0-dev
To compile ppastats:
./compile
make clean all