Git User’s Surveys (2006, 2007 and 2008) did contain version about other SCMs… but this is of course biased. You can check how many people have given SCM in stack at Ohloh, open source software metric site, but I think you can check only current state. There are also results of Debian Popularity Contest (popcon); you can even find some popcon graphs somewhere; here I am not sure whether one can get historical information.
Neither of those sources is a good solution, but I think it is a start…
Added 20-05-2009:
There is also GNOME DVCS Survey results analysis (with links to raw data) on Elijah Newren’s blog (DVCS = Distributed Version Control System).
Added 28-05-2009:
There is Survey on Decentralized Version Control Systems by Brian de Alwis of Department of Computer Science in the University of Saskatchewan being run (see this announcement on Mercurial and Git mailing lists).